I visited a quite normal woodland mansion two months ago. One of the outer walls did not generate, but the rest was working just fine. Spawns were regular and so all the other things.
When I was done with looting and killing Evokers, I set the place on fire, for the sheer fun of it.
Problem is, I'm visiting another mansion now, far away from the first one.
When I got there I was baffled: it was a perfect replica of the firts one, including all the fire damage caused by me and the empty chests I looted. It's as if the game did a copy/paste of the first mansion and put it in a new area. Some parts were still on fire, by the way, just as I left them. In some parts the foundations were visible, didn't look like a "natural" generation.
Is there something I don't understand? I'll try and visit another mansion soon, to see if it keeps happening.
I'm sending some screen shots, thanks for your help!