To replicate the glitch:
Go to an area where the shulker has only the choice of teleporting on the block its on
Spawn a shulker on the block you placed
It will teleport within the block as seen in the image
This works on end rods, chains and lightning rods which all have similar hitboxes which I would guess is the issue.
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[media][media]
Hey there!
This appears to be a duplicate of MC-206707 as this is more active ticket.