I can think of three at the top of my head.
"Dominant Castle"
- superduper rushed towards the end.
"For Your Murder":
- I loved it lots and lots but the end was so luckluster and rushed... it made me not want to re-read, like ever.
But the award goes to....
"If you save an immoral beast"
- the whole ass story A to Z got concluded IN ONE CHAPTER. It was so ridiculous. You were able to tell by the rapid drop in art quality since chapter 20something that the artist gave up on the story early on and just wanted to get it over with lol
I wouldn't bother honestly. It's like Game of Thrones. How things will get wrapped up will leave you empty and disappointed. It started so so good, I even had Inseop as my phone background, something I NEVER did before, because dang boy is so creepy and fine. But he suddenly evolves into an embarrassing, pathetic fool towards the end. That was a 180.
Another commenter described it beautifully:
It's like that picture of a horse that's well-drawn in the first section but then turns into a kindergartener's scribbles.

Is this some new trend? This is the 5th manhwa I'm reading in recent times where we've all enjoyed the story and then there's some angst arc and then it's barely resolved and BAM the story ends like bffr