For as long as the Bedrock version of Minecraft has been out on the Nintendo Switch (not sure about other devices/platforms) the game doesn't allow you to log back into your Microsoft account from the main menu after you've been disconnected (for whatever reason) until the game is closed and opened again. Simply exiting out of the game to the main menu does not work.
It's very tedious to have to boot up the entire game to sign in again and have access to your achievements and the marketplace.
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We're tracking this issue in MCPE-95651, so this ticket is being resolved and linked as a duplicate.
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