Actually jiwoon has not changed at all, folks just didn't notice or remember these scenes...
1Jiwoon telling Suha he fits with his tastes and he should drop all his other sex friends...recalling how Suha left his phone at work and he could see his messages about hook ups
2Jiwoon staying in contact with Suha's sister to gain intel
3 After Suha rejects his confession...after jiwoon rescued him from Red and the drug -selling guy (ch 45)..he confesses again while drunk. His thought bubbles read that even if Suha rejected him again he'd look out for him anyway and keep him caged for only jiwoon to see (chpt 48...his thoughts were chilling af to me)
BUT I also noticed that just before he goes full on darkside Suha brings him back...jiw oon would've beat the hotel ceo/drug guy to a pulp but Suha stopped him. He came to his senses and ratted him out to the cops instead (well played). Suha accepted that second confession and agreed to date him so whatever toxic sht he was thinking was at least put on hold. That incident where he was drugged DID traumatize him as well but hearing Suha say he was gonna confront Red even if grandma was there scared the crap out of jiwoon. He was in panic mode...but then Suha just confessed to him in this chapter. That reassured him a bit, I think. He was genuinely happy Suha said he was important to him.
To all who’s been asking why he changed that much, it’s because of the trauma that the red head gave to him (he’s drugged, saw his lover almost got raped, betrayed by Suha, etc) and he misunderstood when Suha said that once he returns to the company he’ll immediately talk to that red head, Jiwoon thinks that Suha will choose that man over him. I’m not saying that what he’s doing si right but it’s understandable