i try think it just speaks volume of their writing skills. the author improved from Love is an Illusion but they still have so much to room for improvement. I really think that this mother subplot is totally unnecessary and it totally just felt like a device to move Byul and ML love story which contributes to the feeling like Hyesung's story being undermined. It doesn't sit right with me.
They have other plots to give depth on or use like Byul idol subplot and ML background. but they used this so it felt unnecessary.
I definitely agree with you but I do think that this story should be strictly about Byul. If the author wanted to add to Hyesung's story she should have added it in Love is an Illusion. It's like we're moving forward only to go backwards to address the trauma from before Byul was born.
However I do see what you're saying about that subplot being a way to move Byul and the other guy's story forward but the author could have went with the Idol sub plot. It was such a missed opportunity! (╯°Д °)╯╧╧
As a writer I can agree with these comments. If you aren't going to address something entirely in the story just don't bring it up at all. Same thing with a character. If you don't plan on giving them any significance in the story don't give them a name or any comments in the story is an absolute waste. The author could have just not had the mother in this spin off at all if it was going to go nowhere.
This feels like a waste of 2 chapters that could have been focused on Byul and what's his face. The author probably realized that having too many plots at the same time is draining and can often make the story muddy but that's just my opinion