I have signed in to Xbox account. I used to play with this account back in 2016 and achievements were being completed without issues. I recently got back to the game (purchased from Play Store) and after signing in to same Xbox account no achievements are being completed. I also checked with my brother's account and he was getting the achievements (running same version of game on Android 10).
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We're tracking this issue in MCPE-115808, so this ticket is being resolved and linked as a duplicate.
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