This just happened today (March 10th 2021) after the bug fix update came out. Me and my brother play on a world that used to be a relm. It is now a normal world that we have to assess threw his Microsoft account. I join the game (split screen) and I noticed that I lost everything. I spawned at spawn not where I had set my spawn point. I also lost all my inventory, and everything in my end chest. Which was everything valuable I had, and almost 1 years worth of grinding items, for a base. I joined the world like a new player on this world would. I do have my own Xbox so I'm not sure if that had anything to do with it. I just would like it fix so other players don't lose their progress. Thank you.