Repeated biome changes done using World.setBiome() are undone in random chunks when the server is stopped.

What chunks' biomes are reverted is unpredictable; these factors seem to come into play:

• How often the chunks have had their biomes set with World.setBiome()
• Whether the chunk is a spawn chunk or not
• Whether the chunk was in player's view distance when the server is stopped
• Whether a player was inside the chunk during the session

Reproduction

1. Set up a Spigot server with level-type=FLAT, gamemode=1 and a new world
2. Add TestDummy-1.0.0.jar to the plugins folder
   • TestDummy is a plugin written for this issue from this commit
   • Alternatively, use WorldEdit, a large selection and /setbiome
3. Start the server and join it. Observe surrounding chunks are PLAINS biome
4. Execute /test JUNGLE - This sets columns in a 256 block radius around you to JUNGLE biome
5. Execute /stop, then restart the server
6. Join the server. Observe surrounding chunks are JUNGLE biome, as expected
7. Fly a random direction for approx. 100 blocks
8. Execute /test MESA
9. Execute /stop, then restart the server
10. Join the server
    • Expected: Surrounding chunks are MESA biome
    • Actual: https://vanderprot.gamealition.com/img/f91ec.mp4
      • Only the spawn chunks flown through are changed
      • Outside of spawn chunks, only random (unloaded?) chunks outside are changed

Notes

• First discovered on 1.9 live server, reproducible on 1.10 and 1.11 Spigot and PaperSpigot servers using WorldEdit's //setbiome