There's a hell of a lot of times when someone in the community comes up with something, and then the entire community just goes 'yeah, that seems legit' and accepts it to such a degree that they forget where the idea came from. From what I've heard this particular one comes from the roleplaying community; someone in some FC just laid it down as a sort of house rule or something, and because it took until either Stormblood's or Shadowbringers' patches to get an actual counterexample (I forget when exactly Severa got her backstory) it had a LOOOOOT of time to catch on without opposition.

This might be one of the most commonplace ones, but in my eyes it's almost forgivable; this one actually looked legit for the longest time, even if it mostly just got there because for the writers it didn't matter enough to refute. It's way better than 'Black Mages replaced their eye with materia', which is an idea that came from nowhere and is refuted by every single source you could possibly check.