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:

1. Hold a no placeable item like seeds or a stick
2. Go to a blank hotbar slot
3. Hold down left-click
4. Tap the number of the hotbar with the item in it.