I read from a spoiler below that she lives with her Dad and in the later scenes, there'll be a bit of a conflict but she will be reconciling with Sujeong and get along with Gongja as well.
I vaguely remember it being mentioned that she wasn’t great with kids, so she never got close to her daughter and the daughter chose to stay with her dad. But honestly, that still feels really weird and not like a good enough excuse. Even if they’re not close, she’s still a kid. As her mom, Sujeong should be involved somehow, especially considering she spends most of her time taking care of her new son.
That said, since we mostly see things through Gongja’s POV, maybe it’s just one of those things that happens off-screen and the readers never get to see.
Doesn’t Sujeong (Gongja’s mom) have a daughter? It’s kinda weird that she never shows up in the story. Gongja was adopted years ago, and he’s never met or interacted with his sister in any way. I really hope there’s a good explanation for why Sujeong never seems to think about her daughter at all, because it honestly makes me like her a lot less.