After dying, items inside of a temporary GUI slot will be dropped even with the keepInventory gamerule enabled

When the player dies, they will drop items that were put inside of a 'temporary' GUI slot (not a chest, furnace, brewing stand, etc) despite the fact that they are still within the player's inventory at the time, and not permenantly stored inside of a container.

This issue applies for the following block GUI's:

Steps to Reproduce:

Enable keepInventory gamerule before either test

On LAN:

1. Player 1 open a container
2. Player 1 should set an item into a "temporary" holding slot
3. Player 2 must kill Player 1

Singleplayer

1. Set up an 8 block long water stream leading into a pressure plate
2. Place a command block next to the pressure plate (must be activated upon stepping on pressure plate) and use the following command:

    /kill @s 

3. Place one of the listed blocks above in the image near the end of the water stream
4. Walk into water stream with an item in your inventory
5. When possible, open the respective block's GUI and insert an item into one of the applicable slots (do not close the GUI)
6. Observe behavior

Set-up should look something like this:

Observed Results:

The player will drop the items that were inserted into the block.

Expected Results:

The items would not drop from the players inventory, and would instead be returned if enough space is available.