Loss of worlds happens all the time on Xbox One if you uninstall and install the game or remove and add your account to the console. I think it has something to do with synchronization. The worlds get corrupted or simply disappear forever.
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Thank you for your report!
We're actually already tracking this issue at MCPE-84649, so I will resolve and link this ticket as a duplicate.
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I wanted to join a world of a friend, but when I want to enter I always get a mistake, in that world I had already entered, but now he won't let me in anymore