Ever since 1.16 update I have been loosing xps and inventory on my second player. I used to be able to recover them by quickly pulling the plug on my xbox and reloading, but now that doesn't work anymore.
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Same thing on Switch, Switch has guest accounts as well.
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duplicates MCPE-55815, do search!
You may upload your world to that bug report if possible to help diagnose the problem
If your playing this on Xbox One and are using a second player as a guest account its very common.
if you are using another account which has it own unique login then I do not know what is happening their.