Reproduction

1. While creating a new Singleplayer Minecraft world, upload a data pack containing a custom biome
2. Set the Minecraft world type to "Single Biome", and select the custom biome
3. Create the new world and let it load
4. Exit out of the game ("Save and Quit to Title")
5. Re-enter the world and move to chunks that have not been generated yet

Expected

• On first load, the game runs smoothly
• On reload, new chunks are generated as the custom biome

Actual

• On first load, the game stutters (not the main point of the bug, just may be related as this didn't happen in previous versions)
• On reload, new chunks are generated as a regular Minecraft world (for example, could see Jungle and Plains biomes right next to the edges of your custom biome)

Additional Notes

• I double-checked that this issue does not reproduce on 20w30a
• The custom biome I used was a Basalt Delta biome with the modification that Strongholds could also generate (data pack zip attached)
• After reload, before reaching any new non-Basalt Delta chunks, I had regular overworld mobs (zombies, skeletons, spiders, and creepers) spawning in caves in the Basalt Delta.