Put the summary of the bug you're having here
What I expected to happen was...:
I wouldn't be able to place the shulker box where an entity was occupying, especially mobs, items can be displaced but vertically in this situation.
What actually happened was...:
It displaced me as well as other entities such as villagers and thus I can stack them under any entity, (so long as I have enough shulker boxes), as an easy way of raising entities.
Steps to Reproduce:
1. Get a shulker box.
2. Make sure an entity is on a block.
3. Place the box where the hitbox of the block underneath the entity is and the entity should be displaced on top of the newly placed block.
Maybe this is an intended feature like with the piston displacement bug, which I can see as being true.