The bug

When generated, shulkers are, more often than not, incorrectly rotated.

How to reproduce

1. Create a new world in 21w03a
2. Enter the end, run the command /locate endcity, and teleport to the coordinates given
   Observe that most shulkers are slightly incorrectly rotated

Screenshots

In this screenshot, the chunks containing the shulkers has not yet been loaded. This was accomplished by running the command /gamerule spectatorsGenerateChunks false, then running the command /gamemode spectator, and then teleporting to the end city.

In the following two screenshots, the same shulkers are visible after loading the chunks. This possibly makes their incorrect rotation more evident, as it can be more clearly seen which direction the shulker is facing.

In this screenshot, it can be seen that the shulker is rotated on a different axis than the other shulkers shown above. This makes it appear as if it is not connected to the wall on which it has spawned.

Additionally, this issue can occasionally make it appear as if the shulker is intersecting a block, as shown in the screenshot below.