Running along in my base, I accidentally pressed my drop item button ('q'), and my numbers '1' and '2' on my keyboard.  Both items in my hotbar spaces '1' and '2' dropped to the ground and vanished completely.  It was on flat ground, nothing to pick them up, etc.  They just simply vanished completely, and within a few minutes (without picking anything up), they just reappeared in my hotbar.

Was able to replicate the bug by pressing my 'drop items' key along with 2 of my hotbar keys simultaneously.