Every time my brother joins my world (he's on Xbox, I'm on Windows10) he loses items. This only happens with his account, and everyone else can join my worlds fine and never lose their stuff. His character is basically reset, everything he had in his inventory disappears and he's sent back to the original world spawn. The first time this happened, he was holding three stacks of iron and his share of our diamonds. Usually I'd just cheat this back in but I've made him play a world with no cheats so he gets new emotes, because it doesn't always happen, but he has twice in a row lost all his account's save data in this world.