I received a notification that memory was low, and I deleted some old worlds I had to free memory.  After doing this, I found that all of my villagers in a world I had not deleted were gone.  There was no warning that this would happen.  This seems like it is a bug to me, and if it is, I am hoping that it is something that can get fixed for the future.  Specifically, I think that deletion of one world should not affect anything in a different world that is not deleted.