You can change the biome in debug world by "/fillbiome"

I know, this bug will 99% be resolved, but I just want to report it anyways.

As the title said, you can change the biome in debug world by "/fillbiome". This is kinda inconsistent to "/setblock", "/fill" and "/clone" as they will failed. But since debug mode doesn't care about the biome, I think the bug might be resolved.