I read the novel a while back but I think this is progressing differently from the novel..
Iseop's mom actually walks in on them cuddling(?) and iseop is asleep and only mingyeong locks eyes with his mom. That was the turning point when mingyeong decided to leave.
Take this with a grain of salt because I did read the novel a long time ago hehe
Can't say im surprised that had to happen but the painnnnn and angsttt i love this manga though and im on the edge of my seat every time it updates. Im hoping they get a good ending
Spoiler
I wasn't comfy about sakuto and inori ending up together.. they're 8 years apart..