I think bc they started the relationship with one sided love, yiseon didn't truly love her but felt pressured to be in a relationship bc she saved his father's life. He just wanted to 'repay' her that way. Even until now his sister (hakyeong) still feels disappointed when she heard they've broke up and his father even won't let him go back home until he get back with yumin 😭 on eps 23 he said that he wouldn't mind if he saw his ex with a different guy but he also said he would do anything what her ex wants rather having an opinion on his own. So idk if he doesn't have feelings for her. After all 6 years in a relationship is a long time, it's impossible not to grow any feelings towards each other.

As for sojeong, i didn't really like her. she looks so highly on herself, and pretty confident that she can do anything to get what she wants, she's certain that hyeondo will eventually go back to her. And now she wanted to use yumin to gain hyeondo's attention. but we still don't really know abt her back story yet so let's see 😭