End gateways sometimes fail to generate in some worlds

The bug

In some worlds, after the ender dragon has been respawned and defeated once, after any further respawns, no new end gateways will spawn until the world is reloaded.

There also have been reports of end gateways not generating at all in some worlds.

To reproduce

1. Download this end dimension file: DIM1.zip
2. Create a new world with seed 1452395943320050265
3. Close the world
4. Open the world save folder and replace the DIM1 folder with the one above
5. Join the world
6. Build an end portal and enter the end
   → There should be 16 end gateways
7. Summon and defeat the ender dragon
   → A 17th end gateway will appear
8. Summon and defeat the ender dragon again
   → No new end gateway will appear
9. Reload the world
10. Summon and defeat the ender dragon yet again
    → An 18th end gateway will appear