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.
Thank you for your report!
We're actually already tracking this issue in MC-154881, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.