This applies to a few other comments but I'll write it on yours.
Do you know what polarizing means? I'm saying her actual character changes very little but the MC is getting completely different treatment. Affection or not, her treatment of MC is opposite of her actual character (Alpha vs "Submissive"/Loving). Due to this polarizing attitude (opposites) the change to her seems too drastic because of the distance between behaviors, it feels unnatural.
The reason I bring up the physical attribute is because still acts like "whatever nerd" until she discovers it and is sprinkled throughout as a possible swaying factor. My reasoning is due to that fact Roxxy flips the switch and not because she actually changes.
I don't know how explain it because this is character development 101: motivations.
Of course he is getting different treatment, that's how real life relationships work, romantic or platonic, before the route she was a classmate, that's was all there was, she calls him a nerd the same way someone may call someone they don't know fat as an insult but would never do the same to a fat friend or family member. During the route you get to know each other and she heavily depends on the MC, she treats someone she actually knows better, so I don't see the issue that you do.
I can remember the details but I'm pretty certain she was a lot less insulting before the dick thing and even then physical attraction still needs to be a factor for it to be romantic.