I play on the switch, and when joining a world with a friend that I've already played with multiple times before (they use bedrock on the xbox one) it will occasionally have me spawn in at world spawn with an empty inventory and a starter map. All of my items and xp that I had before will be gone and it will be like I join for the first time again. I will have to force close the game and try reloading it several times in order for me to join properly and get all my items back again. It's never happened with my own worlds, only when joining other people's worlds