The bug

While teleporting an entity ~ ~-0.015 ~ I noticed that the shulker randomly teleported until I put a repeating command block down. Then I noticed that they just froze in midair below the minimum block placing height (y<0) then they all went to one spot (and to my knowledge, completely ignoring entity collision).

How to reproduce

1. Create a "The Void" superflat world
2. Place a repeating (always active) command block with the following command

   /tp @e[type=shulker] ~ ~-0.015 ~
   

3. Spawn a Shulker and then look for it below y = 0