Sometimes1

Ms Na is acting with little to no respect for anyone else in this situation.
She's interested in the mansion before they met offered to buy it.
Meets a "handsome lunatic" that calls her by wrong name.
"It's Hae won, not Hae-sun" Ms. Na
SWH apologises.
She develops an unhealthy obsession. Keeps thinking about him after that brief encounter. EP6
SWH apologises 2nd time, gets drunk on shrimp, catches her as she was about to trip .Ep7
Calls him suggests he saved her life invites him to Art exhibit.
His gazes are moving her but he deflects twice on why he does that.
Ms. Na sends a SUV and painting calling him to get acknowledgment. EP8
Meets him for a third time after renovations completed, reminds him of the gifts and asks for a ride.
Flirts with SWJ he's nervous being on a date, teasing him about the relationship with Ms. Joo.
At park surrounded by daffodils she confesses: "You intrigue me, Mr.Seon and I want to know more about you "

He asks if he hug and says "Please wait just a little longer. I have something I must sort out. "
After that, I shall keep our promise, no matter what "
She's unaware of the promise.
Admitting it's driving her crazy, orders Minwah to look into his past, why he lives in that mansion with JIH and everything else related to him. As fast as possible and I'll pay you handsomely. EP9
Minwah returns with stolen drawing of Hae-sun, reports a hidden space in basement. She acknowledges the drawing resembles her, thinks the scroll is hundreds of years old.
Returns to mansion, confesses again she can't sleep is curious, asks for a tour of his space. Who are we to each other?
Do you recall something? EP9
Sponsors a party to confess her feelings.
Reacts very rash after the two hosts dance, threatening them to not toy with her feelings. EP10

2FL channeling her Alex Forest personality?
(Fatal Attraction 1987)

Heartbeat is a romcom of a 400 year old Vampire being bitten, beaten, scolded by the FL.
They have dozens of interesting interactions that ultimately bring them together.

SWJ is not interested in others, his friends noted he's never dated, danced with anyone.
JIH has bitten, beaten,scolded him but a simple praise had him.
She's protecting him (identity) and broke down her walls.
There's dozens of these Rom-com where physical appearances were not the catalyst.
She Was Pretty
Strong Woman Do Bong Soon
Weightlifting Fairy Kim Bok Joo
Fight My Way
Secret Garden
Alchemy of Souls

Here's a few different suggestions
https://kisskh.at/23429-parasite

https://kisskh.at/28134-extreme-job

https://kisskh.at/40459-anchor

https://kisskh.at/685801-big-mouse

These recommendations have have nothing in common with a fluffy romantic comedy.

SWJ is Pinnocho
JIH is surviving, keeping everyone at bay thinking only about money until SWH helps her realise that there's more to life.
These are unfamiliar emotions and reacting by arguing, fighting, celebrating, apologising and moving goalposts in their relationship.

First Kiss EP3
First dance EP10
Movie date
Opened up to each being vulnerable
Nursed her back to Health
Argued, complimented each other
Felt agnst when each other was late returning home
Saved her 2X
Had dozens of interactions including tenderness
Both of them are new to romantic relationships and have exhibited jealousy, desire for each other's company.

Personal Taste was my introduction to Son Ye Jin and responsible for my crush with Korean Rom-Com.
My Sassy Girl 2001 best rom com.
Tropes and cliches are required in these situations.
Abbot and Costello
SWH and JIH
JIH being the straight person allows SWH to showcase hus his comedic talents.

The father isn't dead, moved the gold to a more secure place but became hospitalised, has amnesia from an injury.
Purely wild speculation but these Dramas permit reality to be suspended in many cases.

Shin Har Ri needs Season Two
Shin Hye Sun had me tears laughing in Mr. Queen.
Crash Landing on You Son Ye Jin had my Uncle in tears (his first Kdrama) in Italian subtitles.
Type casting of Park Min Young, Park Shin Hye can be seen as problematic if you care about opinions of others.

Being comfortable in knowing what the audience is willing to accept isn't easy.
Cary Grant, John Wayne understood where they fit on screen.

Jung So-Min, Han So Hee, Hwang Jung Eum, Gong Hyo Jin, Kawaguchi Haruna must be exceptions? Do you think poor writing, directing, casting may have contributed?

SWH was injured fleeing from the ancestor of SDS, a total of five Vampires have been seen so far. Silver tipped arrowhead.
Vampires heal quickly from light lacerations, facial cut in the alleyway.
We've seen open shirt SWH with no visible scars?
Hospital scene was poorly written because his shirt, jacket would have been removed to check for vital signs, heart attack.

1623-1923 SWH full vampire acquires what skill, knowledge? We see him owning a mansion in 1920's Seoul Korea.
Takes nap 100 years and the world is completely foreign to him.
His dumbness to the advancements in the century is relatable. How did he acquire his fortune from running an Inn?
We have a total of five vampires cast, none seem successful (amassed great wealth).
Pens, texting, credit cards etc.
It's been a few months since he's risen.

"I won't allow it from now on."
SWH: "What."
"I won't allow you to leave during work hours"
"I won't let you ditch work even if it's for Ms
Na."
"Got it?"
SWH (Whipped Vamp:): Okay
"And when you stay out, tell me where you're going beforehand.
"Don't just disappear. "
I don't want another person to disappear again.
SWH "I promise I'll do that"

https://www3.asianvote.net/drama/my-heart-is-beating/episode-1

Or you have missed the dozens of moments between the two leads.
Fighting, arguments, rescues, tenderness between them.