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.
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.