The is a vexing contrarianism that happens when someone working in a shared setting seems to be ineluctably drawn to come up with premises that bend if not break rules set up by previous creators. So perhaps the high elves of Lindaria were killed off to an elf during the Demon Wars and their power and wisdom lost forever. Except for this one in hiding for five hundred years. And this other guy. And this one is technically sort of undead. Ooh, and there’s a secret magic interactive library in these caves, and the user interface is totally a hottie. Did I mention my protagonist is actually one-sixteenth Lindarian high elf?
I’m not saying you should never do it, but it seems more prevalent than seems sensible to me. I want to see more “Yes, and” in collaborative worlds and less “Well, actually…” For example: if you want to write about the aforementioned high elves, set your story before the Demon Wars. Or do something exploring in more depth what killed them, and why. Or set a story in one of the dendropolises they left behind. You can explore what’s interesting about the elements of a shared setting without undermining them. And you should.