I think he knows there's something wrong though
Does he really love Yiseon? If so how can he didn't see Yiseon's depression/suffering. In my opinion, Hyeon just think Yiseon is some kinda important property that he needs to have.
He knows Yiseon has depression and tries to help him with that (be it for selfish reasons or affection) but since the chapter before, Hyeon thinks he has Yiseon in the palm of his hand and didn't even consider that he would try to die again. He thinks he helped Yiseon 'get over it's by making him dependant on him
Yeah, I think, for now, he doesn't really understand how he feels about Yiseon. That's why even if he knows there's something wrong with how the MC acts right now, he doesn't really actively try to find out about it because he thinks MC is in the palm of his hand and he can just manipulate him later on.
ML's so smart normally. How could he not have seen this coming from a mile away?
Love really is blind sometimes.