Good theory, it makes a lot of sense
Someone waaay back, who seems to have read the novel, said that they were hookup partners once, and just when redhead started liking Jeongyoon, he said that he was interested in Hyeonsoo, so redhead snatched Hyeonsoo's crush as some sort of revenge.
I read this MONTHS ago, and I don't know how to find the translated novel unless I pay for it, so take this with a grain of salt because I don't even know if I'm remembering well.
wow, that's next level assholery
Dang! So the asshole is not Jeongyoon. I was worried he will become a red flag seme.
This is just my hunch...
What if Jeongyoon asked/colaborated with the red-haired guy to make Hyeonsoo's crush his gf so Jeongyoon can take Hyeonsoo himself?
It seems the red-haired guy knows Jeongyoon likes Hyeonsoo.