I don't understand why are they using she/her for Hyesung, so annoying.

I know right? I thought they call him mom but with he/him so I was confused. Maybe because he's an omega they can use both?

I think it's just mistranslated, cause previous chapters, and even in the main story they would refer to Hyesung using he/him, and call him "mom"

Yeah, it's probably mistranslations since there's mostly no gender differences in talking Korean(and most asain countries), like she/her he/him in English