When using the number keys to swap from a 'weapon (including things like flashlight and compass)' to another 'weapon,' or when swapping from a 'weapon' to an inventory item like bandages or a water bottle pressing the assigned key merely unequips the weapon, rather than unequipping and then switching.
Addendum: Seems to only happen when crouched or prone.
Place a variety of items in the number key slots, including guns, food, bandages, water, flashlight, and compass. Press number keys to try switching among them. Try in all three postures.
Swapping from inventory item to inventory item, as from bandages to a water bottle seems fine.
same here, but it sometimes seems to be because of a lag between key entry and the action.