Yeah fuck I am like 2 years late to this game but from what you've said it sounds like I should avoid this game too, I agree a LI in a game is an LI from the start, it doesn't matter if she isn't in a relationship with MC at the start of not the point is that eventually sooner or later said girl will be involved with the MC, that makes her an LI and so yeah I would lose all emotional investment as well.
Women in games are either a love interest or they aren't, there is no "She wasn't a love interest at the time" a love interest doesn't define her relationship, a love interest means she will be a character of interest for romantic reasons.
If you good sir have managed to keep playing to this day please let me know if that scene or whole situation has changed, if not I'll avoid this game because yeah it's basically NTR or if it isn't NTR it gives the same negative feelings