`FlatLevelSource` (Flat World ChunkGenerator) caches the generation settings causing any Neoforge Biome Modifications to not work

[pietro-lopes]

Minecraft Version: 1.21.1

NeoForge Version: 21.1.217

Steps to Reproduce:

1. clone this repo https://github.com/pietro-lopes/FlatChunkGeneratorBug
2. Create a world with commands/cheats ON
3. Close the world/game
4. Launch game, enter previous world
5. teleport yourself using /execute in flatchunkgeneratorbug:broken run tp @s 0 256 0
6. fill with air to peek the terrain for ores /fill 1 252 1 9 -63 9 air
7. You should not be able to see any ores
8. teleport yourself to a working dimension where it clears the settings cache /execute in flatchunkgeneratorbug:working run tp @s 0 256 0
9. fill with air just like before, you should see a few ore veins

Description of issue:

When you have a custom dimension with chunk generator type minecraft:flat and you use Neoforge Biome Modifiers to add stuff, those modifications are not applied in certain conditions because how FlatLevelSource caches their Settings

Example:

• Create a new world, and visit that dimension, works fine for that session.
• Join a previous created world runs a ChunkGeneration#validate which triggers the caching
• Using "Recreate world", also causes same issue

Some discussion here:
https://discord.com/channels/313125603924639766/852298000042164244/1455623815186087956

[neoforged-releases]

🚀 This issue has been resolved in NeoForge version 26.1.2.7-beta, as part of #3086.