Undead mobs not attracted to turtle eggs above foot level

If you place a turtle egg on the ground in a flat area and then spawn an undead mob, the go stomp it. But if you move the egg up one block, so that it’s at head hight, the mobs don’t care. 

how to recreate 

1. place an egg on the ground.
2. spawn an undead mob.
3. move the egg up a block and compare mob reactions.