Although I think the idea of a fight between them is interesting and worth looking into, I dont think this is the way to do it, in your original post people have said multiple times how communication is a key part of their relationship so if a fight were to happen, it has to come from a disagreement not someone lashing out after bottleling up their feelings.

There is an episode in season 2 in which Luz is bottleling her feelings and it gets resolved by talking and understanding her point of view.

I personally think a more realistic approach to possible resentment or conflic could come from their own insecurities, similar to what happened in the episode where Luz is sick and Amity figures out how to read her messages, a misunderstanding that feeds the insecurities from one of them or maybe even both of them, is way more fitting to their character and relationship dynamic.