The player's off hand slot does not update visually while a GUI is open

Any change to the item stack held by the player in their off hand slot does not cause it to update visually on the client if a GUI is open, until the GUI is closed.

How to reproduce:

1. Give yourself a few of some type of item and stack them together in your off hand slot.

2. Place a chest.

3. Drop one item from your off hand slot and quickly open the chest.

4. Wait until you pick the item back up.
   -> ❌ The number of items in your off hand slot doesn’t update.

5. Close the chest’s GUI.
   -> ✔ The number of items in your off hand slot updates.

Expected result:

Changes to the player’s off hand slot would always display on the client immediately.

Observed result:

Changes to the player’s off hand slot do not display on the client if a GUI is open, until the GUI is closed.