When a player re-joined my world (via a LAN game which is how we always play), any progress they had made was completely lost. They lost all their items (including anything I gave them) and all their experience. They also spawned back at the main spawn point of the world, it is as if they are a brand new player who just joined.

We both tried logging out and back in again but it did not work. I am playing the game on Windows 11 (host) and they join while playing on Xbox One.