Ghost items can be created using /item (server doesn't update client inventory correctly)

Using /item to replace the item in a player's hotbar slot frequently creates ghost items. Right-clicking with a ghost item will still place a block, but it does not show up in the hotbar.

This did not happen in 1.16.5.

Steps to reproduce

1. Set up a repeating command block with the following command:

   item replace entity @p hotbar.0 with dirt
   

2. Drop the dirt block
   → The slot will become empty, even though the /item command should refill it
3. Press Q again while selecting the first slot
   → You'll drop dirt, even though there's no dirt in that slot
4. Right-click onto a block while selecting the first slot
   → A dirt block gets placed

Attachments

• Quick demonstration: Minecraft 1.17 Release Candidate 1 - Singleplayer 2021-06-05 14-59-59_Trim.mp4
• Slime blocks turning into a ghost item for the first time (1:50)
• This clip showcases buggy offhand behaviour (1:09)
• A brief encounter with ghost items (0:19)
• This clip contains some interesting events that may have caused the ghost items (1:15)
• This video shows that death fixes the ghost item (0:18) (~3mins cut out)