When you modify biomes that appear in the Vanilla-Layered set (such as Plains) in custom terrain generation, it generates unmodified biomes that are set to "null".
I provided a datapack (bug-showcase.zip) to show the bug. It turns all would-be plains biomes in a Default world into "null".
I’ve had this happen, but don’t remember how it was formatted. These custom biome worlds don’t support the vanilla_layered type, which is a bug.