There is a bug when you hold an item at your last hotbar then go to chat or seeking inventory (in android), it'll automatically switch to previous hotbar. It happened to me at 1.16 full release until 1.17.40 now.
MCPE-145398
There is a bug when you hold the last item in your hotbar, and if you do chat or seeking inventory, it'll automatically switch to previous hotbar
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We're tracking this issue in MCPE-49117, so this ticket is being resolved and linked as a duplicate.
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