More serious/thoughtful drama with a good love story? Lost (human disqualification) or My liberation notes. Don't think they met in the past. A piece of your mind is another good one. These are the first ones that come to mind.

Old but worth it Yoo Na’s Street

Ooooh, interesting! Ok, serious/thoughtful, let’s see what we got. I’m going for variety here and maybe something will stick.

The first one that comes to mind is Mr. Sunshine which is *ahem* definitely not a rom com. The leads have a pretty big age gap, so ML had already left Korea by the time FL was born, so no childhood connections here! Bonus because their meeting is super great and dramatic.

The next one I’m going with is Search WWW. I don’t think any of the three FLs has a past connection with their love interests. The main romance doesn’t do a lot for me, but the one between Hyeon and Ji-hwan is something I go back and rewatch frequently, as is the drama that he stars in.

*Never not plugging Live Up to Your Name* Third one I recommend is Live Up to Your Name! This one is a time-slip, so there is no reason for them to have met in the past because he’s already from the past! There is a very minor connection between a previous time-slipper and the FL, but it is not even slightly romantic. Also, this drama is just great. Kim Nam-gil making me snort with laughter and ugly cry in a beautiful cycle.

There are probably lots more, but I have to stop somewhere, so I’m ending with Unlock the Boss. I don’t know how serious it is, but it’s not a traditional rom com, and parts of it are quite thoughtful. The ML is pretty much the best thing ever and the FL, with all her conflicting loyalties, is quietly great.

Thinking of these romances.....agree or not?
Descendants of the Sun?
Youth of May?
Business Proposal?
Because This Is My First Life?
Weightlifting Fairy?
My Girlfriend is a Gumiho?
The Master's Sun?
Beethoven Virus?
The Inheritors? (does meeting in California count)
Encounter? (does meeting in Cuba count)
Padam Padam?
1% of Something?
She Would Never Know?
Sorry, I Love You?
Personal Taste?
My Lovely Samsoon?
Full House?
You Are Beautiful?
Strong Woman Do Bong-Soon?
Another Miss Oh?

Would 'Run on' count?
Do you like Brahms.

I don't think of these as romcoms.

Kidnapping Day (because the prompt says "love story" not "romance" :)

Doona (unless you think "love story" means things have to work out)

*bursts through wall a la Kool-Aid Man*

BECAUSE THIS IS MY FIRST LIFE

Search WWW

All 3 female leads did not have a past relationship with their love interests.

Ta-Mi and her much younger lover met by chance.

Ga-Kyeong was in an arranged marriage and somehow developed feelings for husband

Cha-hyun met Ji-Hwan because he played the antagonist in her favourite daily makjang drama

Do you mean past connections as in childhood past connections or any connections whatsoever - highschool, university, workplace?

Forest of Secrets/Stranger.
My Liberation Notes - One of the make leads only came across one of the female lead's voice but not a physical face meet.
Because This Life Is My First.
Business Proposal
Crazy Love.
Ruby Ring.
Mr. Queen. Technically, Bong-hwan was meeting Cheol-jung for the first time in So-yong's body.
Moon Lovers : Scarlet Heart Ryeo. Same time travel hijinks as Mr. Queen's.

I believe that The Best Hit, Flower Boy Next Door, Love to Hate You, and Dali and Cocky Prince should all fit the bill! (*to the best of my memory*)

To the best of my recollection:

Alone in Love
Go Back Couple
Our Blues (HJM & KWB)
When the Camellia Blooms

One Spring Night

1. They already know each other —because they're already married/divorced—, but no childhood connections or reincarnation:

Matrimonial Chaos
Flower of Evil
Cunning Single Lady
Once Again
Fate and Furious

2. Cheating isn't that bad? Nope. It is bad, BUT...

Love Affairs in the Afternoon
On the Way to the Airport

3. Grown up vibes but with a little bit of comedy

Coffee Prince (the secondary couple is everything)
Yu Mi's Cells
1% of Something
Because This Is My First Life
It's Okay It's Love

Goblin - he never met her or laid eyes on her before walking past her under the umbrella.

Shopping King Louie

Extraordinary Attorney Woo

My Girlfriend Is A Gumiho

Sh**ting Stars

Backstreet Rookie

The Interest of Love

Coffee Prince

Greasy Melo

Reach of Sincerity

The Master's Sun

Secret Garden

Warm and Cozy
Flower Crew: Joseon Marriage Agency

Poong, The Joseon Psychiatrist

Joseon Attorney

Boys Over Flowers

Extraordinary You

Secret Royal Inspector & Joy

Hundred Days Prince

My Sassy Girl

Moonlight Drawn by Clouds

Strong Girl Bong-Soon

Witch At Court

Behind Your Touch

Yong Pal

Live Up To Your Name

Business Proposal

Little Channel A’s TWELVE NIGHTS (2018) (Viki US) (12 eps natch). Episode one was filmed in NYC including a few seconds at my local subway stop in Queens. If you would like to see midtown Manhattan pre pandemic this is the place to see it.
TN made me a fan of 6’ 2” ML Shin Hyun-soo who portrayed a contemporary dancer. I enjoyed FL Han Seung-yeon but she has appeared on only one drama since. Ye Soo-jung was a joy as a professional photographer rather than being cast in her bread and butter roles of mom/grandma. I wish she was cast in more roles like the one she had in TN.

She Would Never Know> It's not a rom-com but a very good romance.

Coffee Prince> I don't remember they had a connection before.

My Liberation Notes
Law School
Undercover
Touch Your Heart
Find Me In Your Memory
Gaus Electronics

Hyena. It’s the answer to every Kdrama quiz, including this one. 😉

Also:
Are You Human, Too?
Oh My Venus
Fated to Love You (Despite the title, they had no childhood connection.)
Secret Love Affair
A Witch’s Romance
Sungkyunkwan Scandal
Oh My Ghost
Another Oh Hae-Young
I’m Not a Robot
Jealousy Incarnate
Secret Love
Protect the Boss

My problem here is that I can totally forget these sorts of things, so I'm all, "Did Hello, Me! have a childhood connection?" I don't think so--not between the main couple? Or did they? Oh, who knows. Oh, right, yeah, they did because he was younger...oh right wasn't there bullying?...and then I get disappointed that they really do all have this trope.

It happens some of my favorite dramas don't have that childhood connection:
Flower Boy Next Door, Live Up to your name, At a distance spring is green, 25-21, Start-up, The Best Hit, Yumi's cells, Fated to love you...

Crash Landing on You
Secret Garden
Behind Your Touch

Scent of a Woman

Piece of your Mind.
Arang & The Magistrate.
Call it Love.
Goblin (he was waiting for someone he didn't know).
Bossam.

My fav shows are usually ones without the childhood connection trope. Apart from shows other beanies have mentioned, below are some:

Misaeng
Wok of love
Life
Mad dog
Watcher
Signal

The title of this post made me chuckle.. xD

The timing of this post made me think DB Staff is throwing shade at Castaway Diva, then I realized it feels like 70 - 80 percents of dramas recently and to come have this “past encounter” tripe it’s almost become a syndrome.

For thoughtful romance from recent dramas I suggest Call it Love and My Liberation Notes. Is It’s Okay, That’s Love falls into rom-com category? It had a lot funny moments but I think it just a side dish compared to it major theme of love through mental illness which very heavy, important and thoughtfully portrayed.

Run On!

Memorist
Life on Mars

BECAUSE THIS IS OUR FIRST LIFE is fantastic and the OTP Se-Hee and Ji-Ho have definitely never met as children. Neither did the two side couples.

The childhood link is on the women's side as Ji-Ho and her two friends were classmates in high school.

The male characters never knew each other as kids but they all work for the same company and two of them went to college together.

Age Of Youth S1&2
Let’s Eat S1
Welcome To Waikiki S1 (the leading romantic couple haven’t met before)

Strong woman do bong-soon
The heirs
Crash landing on you
Record of youth
Yumi's cells

Since I just did a rewatch, Lovers in Prague.

Tired of this trope already. What about meeting new people? Or the fact that first love rarely lasts?

I don't really watch rom-com lately, but one that I remember enjoy watching is Marriage Without Dating. The story is fun and the OTP is very likeable.

As for heavy series, I won't ever shut up about these:
Forest of Secrets, Life, Dear My Friends, My Mister, Misaeng, Prison Playbook, and Kingdom, and my recent favorite: Happiness.

Though it's a 2023 trend, most of the drama I have seen have no childhood/adolescent/past connexion. I was surprised when I ran the numbers. Maybe I've missed/forgotten a few backstories.
In order to present a nearly reasonable list, I've excluded dramas with important romance or comedy.

Taxi Driver 1&2
Reborn Rich
One Spring Night
Once upon a small town
Memorist
Tell Me What You Saw
Do you like Brahms?
Summer Strike
Bad and crazy
The Innocent Man
Stranger
Pinocchio
If You Wish Upon Me
Start-Up
Save me
Uncontrollably fond
Are you humain too?
Tunnel
Cheese in the trap
Now we are breaking up
Flower of Evil
Defendant
Negotiation
Liar game
Docteur John
Encounter
One sunny day
Kill it
Descendants of the Sun
Memories of the Alhambra
It's ok not to be ok
The K2

My favorite would be:

Strong Woman Do Bong Soon
Behind Your Touch
Extraordinary Attorney Woo
One Spring Night

Run-on.
Chief Kim (and I really mean chaebol boy and Kim, but I don't think the FL has met him before, either. And Junho and him only as grown-ups).

Lost.
Can't believe I forgot that. It's one of my favorite k-dramas.

Love Rain comes to mind first. Neither of the couples that Yoona and Jang Geun-Suk played met as children.

Touch your heart/ Reach of Sincerity was second for me. Unless you count them meeting in Goblin in a past life. 😂

Third one I thought of: The Secret Life of my Secretary.

Fourth through sixth: Romantic Doctor Teacher Kim— 1,2,3

Most recently, MOVING. None of the older super power couples met as kids. Of course, all their kids are meeting as teens. 😂

Flower Of Evil - one of my all-time favourite Kdramas.

Crash Course In Romance - loved this, too!

I feel like a weirdo, because I adore the past connection trope so so so much. It's one of the reasons I love k-dramas. HOWEVER. I also like the ones where the OTP never met. One of the things that makes Start Up so great is the fake-out. I think I get a bigger charge out of it because I do like it when they have met before or are first loves.

Some others:
Woks of Love
Marriage Not Dating
Because This Is My First Life
Search WWW
The Beauty Inside

Well, you probably found a lot more already!

I do want to make a pitch for the plot where they do know each other already and also, they didn't have like, amnesia or something. Drama writers count on how different the younger and older actors look to account for why characters don't remember each other. Or sometimes they have contracted face blindness as in My Holo Love (but it doesn't work like that!) or they have some other major issue to obscure that it's the same person. It's much cooler when they remember each other all along.

Alchemy of souls - FL met the SML as a teen but not the ML.
Bring it on, ghost! - FL met the ML's father (?) once but never met ML directly.
Bora, Deborah!
King the land. (they meet in ep 1 and then they skip a few years ahead to continue the story.)
Dali and the cocky prince.
Cheer up.
Be Melodramatic.
Behind your touch.
Descendents of the sun.
Touch your heart.

I don't mind a childhood or past connection or lack of as long as the story is good. Some more without a past encounter:

My Mister (not a romance but the main leads had no shared past)
Coffee Prince
Secret Love Affair (had a sort of simultaneous pen-pal connection but no meeting and occured around the same time that they first met)
Business Proposal
Itaewan Class
Witch's Romance
Secret Romantic Guesthouse
Mr. Queen

And then there are the ones where the leads met as children but remained friends for many years before their relationship changed, so they shared a past but no amazing/coincidental/reincarnation reconnection years later:

Fight My Way
Romance is a Bonus Book
Answer Me 1988

I completely went ahead of myself and was thinking about drama leads who never met each other in the main story and not that they had never met before the main story LOL So the drama that immediately came to my mind is “A Piece of Your Mind”— the FL & the 2nd FL never met the ex of the ML— but I guess this drama meets the 2 things that I mentioned 😅