Spawned item entities from a mobs loot table may not save their "Item" tag resulting in the item entities getting killed immediately after they have spawned. I have noticed this happening with the loot command. I don't know if the same applies to a natural scenario.

Reproduce: 

1. Spawn in any mob whose loot table can drop items (e.g. creeper)
2. Setup a command block chain:
   1. Impulse
      Set to: "Needs Redstone"
   2. Chain
      Set to: "Always Active"
      Command:

      loot spawn <pos> kill <the mob entity>

      e.g. after placing a witch near the command block:

      loot spawn ~ ~1 ~ kill @e[type=witch,distance=..5,limit=1]

1. 1. Chain
      Set to: "Always Active"
      Command:

      execute as @e[type=item] unless data entity @s Item run say hi

      Make sure the @e selector won't catch any other existing entities.

1. Activate the impulse command block (probably multiple times)

You may notice the "Item" tag sometimes being omitted in the chat message.