The bug

Shulkers teleport after generating, causing them to be positioned incorrectly when the player reaches the end city. This is caused by the fact that shulkers now generate one block above where they used to generate.

Well-known example

In previous versions (1.16.5), a shulker always spawned in front of the block between the two chests:

However, in 21w03a and above, the shulker spawns one block above where it previously generated:

This causes it to instantly teleport when the chunk is loaded, causing the shulker to be placed in a completely different place than it is supposed to:

How to reproduce

1. Create a world in 1.16.5
2. Enter the end (/execute in the_end run tp @s ~ 100 ~)
3. Find an end city containing an end ship (making use of the /locate endcity command may help)
   A shulker generates in front of the block between the two chests, as intended
4. Create a world in 21w05a
5. Repeat steps 2-3
   The shulker is not in the same place, as it has teleported as soon as it spawned

Additional information

This issue clones MC-213542. That report was resolved as Incomplete by slicedlime, as the expected behavior, and what the issue really was, were not present in the description.