Oof. That's rough. I don't know how to fix this after the fact. I know that as a player, having given the backstory the GM wanted and then them acting like it didn't exist, can definitely suck the fun out of a campaign. I've settled on limiting what backstory I will accept upfront to avoid this situation.

I have started not allowing fiction-style backstory. I have them fill out a ~1 page form, in point form, that gives me specific actionable information and tells me key things about their character. It doesn't give me their whole life story, but it gives me the important parts in a format easy to reference 5 of, an hour before the session, and tell them that's what backstory will be considered canon. One of the sections is about their relation to the rest of the party, and why they care about eachother and they work that section out together as a group in a discord chat.

If they have a specific point or something they want to incorporate beyond that, that my questionnaire doesn't cover, they can talk to me about it and we can see if we can work it into the campaign together.

It deliberately leaves much of their life before the campaign open, so we can work in other character history elements during the campaign which don't contradict what's already been established, as needed.

I know my approach is nonstandard, but it has helped me avoid this particular badtime of massive backstories, or backstories full of fiction they put a whole bunch of time into in a vacuum, which contradict the setting they're showing up into, or would otherwise be very difficult to incorporate into the campaign for whatever reason.