When holding down the use button on a Bluetooth connected mouse and using a keyboard to switch inventory slots the game cannot decide what to do. Unless there is something placeable it will not work but you have to be selecting/outlining a block.
Steps to reproduce:
Hold a no placeable item like seeds or a stick
Go to a blank hotbar slot
Hold down left-click
Tap the number of the hotbar with the item in it.
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We're tracking this issue in MCPE-135437, so this ticket is being resolved and linked as a duplicate.
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