Custom/modified biomes don't work in the overworld

Trying to modify any pre-existing biome in the overworld won't load the biome, and trying to load any custom biome not in a custom dimension (or in single biome worlds) causes the same error. The log gives the warning "Unable to find spawn biome" and then spams "Recieved invalid biome id: -1" (I got over 70,000 of these in less than a minute) on world creation when trying to join your new world. There are no error messages, only warnings.

To reproduce this, try to load any custom biome in a single biome world, or modify any existing biome in the default overworld and try to load up a world (I tried putting a snowy village in a plains biome to get this to work).