When I have a multiplayer world open, if another user tries to open the world it will sometimes open them into a completely different world. On both iPads it shows a message showing that the new user has joined but they are not in the same world on screen.
Once this has happened the second user is never able to gain access to the world again. Not all worlds shared by the two users are affected.
Two different worlds have been affected from a large number hosted on each iPad.
Quitting minecraft and re-opening and rebooting the iPads has had no effect.