Whenever you start your game, the skin you where using becomes ether Alex or Steve. This does not affect your other skins only the one you last used. It is very annoying to have to recreate your skin every time you play the game and I hope this gets patched in the next hot fix.
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Thank you for your report!
We're actually already tracking this issue at MCPE-53398, so I will resolve and link this ticket as a duplicate.
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