Summary:
Items dropped upon death lack the 'Thrower' NBT tag.

Steps to Reproduce:

1. Have one item in your inventory
2. Run the command /kill @s
3. Return to the spot where you died (without picking up the item) and run the command: 

   /data get entity @e[type=item,limit=1] Thrower
   

Observed Result:
This will reveal that the item does not have a Thrower tag, despite technically having been dropped by the player.

Expected Result:
This will reveal that the item has a Thrower tag.

This is related to MC-96782 but is not a duplicate, as MC-96782 does not mention that this bug occurs upon player death.