I always wait for Xbox to login before I play because I know that if you don't, it uses a different local account, yes. I also always go to the main menu before closing out as a habit so I know neither of those are the issue. Maybe it's storage space or something as I know my PC has low storage capacity so I'll have to look into that.
I have experienced this issue on two worlds that I have spent countless hours on, and I can’t believe this has not been addressed since those worlds were affected. Since it seems to affect the larger worlds, this will only continue to be a problem as people spend more time playing. Going into a game knowing this, how are you supposed to enjoy the game while being aware that it will eventually be unplayable? This is incredibly unfair and makes the game pointless until it is addressed directly and corrected appropriately.