Trying to load any biome introduced by this mod with the buffet generators will show the chunk generation square thing correctly up to 100%, the player will join the singleplayer world, but once it gets to the
Saving chunks for level 'ServerLevel[World Name]'/minecraft:overworld
part, it spams
Received invalid biome id: -1.
The server workaround of deleting the "region" folder doesn't fix this.

Log file

MC version: 1.16.4
Mod version: 1.0.2

Current workaround is to add the datapack before world creation. The mod's biomes aren't meant to be used for these world types so there will be various issues.

Fixed with release v1.0.3.