No spoilers as I haven’t seen the movie yet… so there’s quicksilver and the scarlet witch. Both mutants and written as magneto’s children for decades. Also avengers characters for decades. Marvel attempted to essentially create a backdoor and steal their characters back from fox (all mutants are fox property in multimedia).
Fox was within their rights to object to this, from my pedestrian understanding. And fox likely saw the battle for quicksilver for what it was: a transparent ploy to destabilize their claim to the x-men characters as a whole. Citing source material is an insidious argument because those characters had been intermingled for decades but the actual licensing agreement has always been fairly straightforward (from an outsider’s standpoint)
Changing quicksilver and scarlet witch’s evil father from magneto to strucker is probably the most insulting detail switch because strucker already had his own boy-girl twin children with superpowers. It’s actually kind of maddening. But the obvious difference is that in the source material, the strucker twins are evil and quicksilver and scarlet witch become avengers.
Fox also had access to a super-speed mutant who has a bad attitude who incidentally also has a twin sister. There was no reason why quicksilver and the scarlet witch needed to be the battleground over which fox and marvel should go to war. No reason besides that marvel desperately wanted to reclaim what they once sold off, no matter what. Marvel logic probably works like: if they could sneak quicksilver back, they might be able to hammer out some sort of deal for the popular Hugh Jackman/Wolverine combo. That was perhaps their long-shot hope.