The bug

The client is not reliably informed when items are added to or removed from the player's new saddle and armor.body equipment slots. As a result, certain components that rely on the client work incorrectly.

Swapping the item into place by using it does update the client, but placing it with /item does not. Dying does not update the client until respawning.

How to reproduce

1.

/item replace entity @p armor.body with diamond[equippable={slot:"body",camera_overlay:"misc/pumpkinblur"}]

2.

/give @p stick[equippable={slot:"body",camera_overlay:"misc/forcefield"}]

3. Use the stick and diamond
4.

/item replace entity @p armor.body with air

Expected results
The pumpkin overlay should appear after the first command, and disappear after the final command

Observed results
The overlay is not updated unless the item was used manually by the player

Another example

/give @p stick[equippable={slot:"body"},glider={}]
/item replace entity @s armor.body with air

Equip the stick, then run the command. The player is able to start gliding, since the client still thinks the item is equipped, but the server immediately cancels it.