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
Create a "The Void" superflat world
Place a repeating (always active) command block with the following command
/tp @e[type=shulker] ~ ~-0.015 ~
Spawn a Shulker and then look for it below y = 0
Thank you for the report but one screenshot would have been probably enough