When a shulker teleports through a nether portal, their rotations gets all over the place.
Video demonstrating the issue: https://youtu.be/lyCt_jcWBsw
Shulkers teleport with original dimension coordinates after passing through portals
Shulkers get rotated when upgrading to 21w03a
Shulkers are spawned slightly mis-rotated after a shulker was hit by another shulker's shulker bullet