Using the 1-9 keys to move blocks in the hotbar break their placeability

Get a block, any block, out of the inventory and hover your mouse over it, then press any key that corresponds to the hotbar. Pretend that the block was in slot 0 press the key for slot 3 (for example). Now try and place that block down and it will render the block or less than a second and then it will despawn and fail to place