Entities can't be seen through spectator ghost head.
Steps to Reproduce:
1. Spawn an entity.
2. Go in to spectator mode and press your toggle perspective button. (Most commonly known as F5).
3. Line up your head with an entity and you will notice it disappears.
Entities aren't visible through the spectator ghost head.
You should still be able to see the entity.
I've attached one image which shows this problem.
This works for all mobs and all dropped items.