Generate a superflat world using biome ID 128, something like: 3;minecraft:bedrock,2*minecraft:dirt,minecraft:grass;128;village
Load the world and press F3, it shows a biome of "Ocean" instead of "Plains M".
Is this a bug? Or is the wiki wrong about the biome IDs?
This might be a different issue entirely, but one more piece of evidence that there is something weird about the biome "Plains M": I was using a utility called "Biome Painter" which lets you reassign biome IDs to different map regions, and added a "Biome M" region (id 128) to the map. Standing in that portion of the world, F3 simply shows "Plains" and not "Plains M".
So either the F3 is incorrect, or there is something else weird about 128-Biome M. Incidentally, I checked all 57 other biomes, they all showed correctly in F3.