Shulkers get rotated when upgrading to 21w03a

The bug

When a world containing shulkers is upgraded from an earlier version to 21w03a, shulkers on the ground will get rotated.

How to reproduce

1. Create a creative world in 20w51a
2. Spawn some shulkers on the ground using spawn eggs
3. Exit the game
4. Load the world in 21w03a
5. The shulkers will be rotated