Please don't accept her suppositions out of hand. If it's not the man himself telling you his feelings, then a person who is not even close to him probably doesn't know the first thing she's talking about. (The FL is a dumb. She's very intelligent except for when it comes to feelings.)
The Prince married Maryjun for love in the previous timeline...HOWEVER, he only became concerned about her because her older sister bullied her out of obsession for him (thus gaining his attention). As there is currently no bullying, he's not concerned about Maryjun...she's just a cute, accomplished girl, and the younger sister of an acquaintance whom he DOES currently have an interest in (because his stalker suddenly started avoiding him, and no longer behaves as he would expect her to.)
What you're saying makes a lot of sense and I think I overreacted before because I really loathe the character but thanks for clearing up the situation!
what kinda scumbag would just marry another girl's sister to spite her? I thought he really married her bc of love but that prince doesn't deserve shit