What I expected to happen was...:
That the shulker would not move.

What actually happened was...:
The shulker moves one block down and when you try to stand on it, you start to glitch out (shake up and down). Re-logging solves the issue.

Steps to Reproduce:
1. Place down a command block with this command:
/summon ArmorStand ~ ~1 ~ {Invisible:1,Passengers:[

],NoGravity:1}
2. Activate the command block and fly atleast 80 blocks away, so that you can't see the shulker anymore.
3. Fly back and notice the shulker has dropped down one block.