When respawning the ender dragon, you need 4 end crystals in certain positions. The last end crystal must be placed directly, not pushed into place by a piston or spawned in using the /summon command.

How to reproduce

1. Place 3 of the end crystals needed to respawn the ender dragon, in the appropriate locations. Leave the north or south side of the portal empty.
2. Place the fourth end crystal one block west of the necessary location. Placing it 1 block east will respawn the dragon immediately, but that is a separate bug.
3. Push it into place with a piston.
4. Destroy the end crystals. Repeat step 1, but this time leave the east or west side empty.
5. Place the last crystal one block north of where it should be.
6. Push this crystal into place.
7. Destroy the end crystals again, and repeat step 1. This time, it doesn't matter which side you leave open.
8. Spawn the last crystal using the /summon command.

Expected result
The Ender Dragon should respawn when you complete steps 3, 6, and 8, since there are 4 crystals in the right configuration.

Actual result
The ender dragon does not respawn when you complete any of the steps. As far as I can tell, the last crystal must be placed manually.

My assumption was that every tick, the game checks whether there are end crystals in the right configuration. The game processes end crystals each tick anyway, so this should not be a big deal to implement.