I'm gonna take a more "positive" route and think that this is definitely a misunderstanding problem.
From the quote small lies cause problems,
so from that small lie of not being able to come eat the stew because of group project -> hyeonseon found out it's a lie, there's no group project -> probably more yeoreum's dark past uncovered when yeoreum tried to hide it causing more misunderstanding -> hyeonseon thinking yeoreum aren't satisfied with him and attracted to other guy -> to this last scene where he thought there's smth between yeoreum and this guy.
The brown hair guy is obv not the guy that's obsessed with the brother so i don't think it's a sequel of it. So i went to twt to find more information and someone said that this new manhwa takes place in the 90s so not a sequel at all. Sad that it means we are not getting more jooin and our mittens content more, but at least the author won't focus on the shitass rapist brother











When dawner says "when did i said dee had horns" his face thoo like bro calm yourself, you about to turn into those obsessive ml