It turns out this plains m biome is actually the roofed forest m biome.
Roofed forest m biome id.
Main biomes have a numeric id from 0 to 127 while biome variations normally have a numeric id of 128 original biome number.
I just attached the pictures that shows the biome id of the biome.
Renamed mutated roofed forest to dark forest hills.
Biome ids got changed in 1 13.
Roofed forests are biomes that were implemented in the update 1 7 2 along with many others.
Added lily of the valley that can generate in dark forests.
This means this is now a bug report on roofed forest m biomes not generating trees or tallgrass.
Added roofed forests along with many other biomes.
Changed the id roofed forest to dark forest.
Renamed roofed forest m to mutated roofed forest.
Mountain edge deep.
Renamed roofed forest to dark forest.
These biome numbers are used when creating a customized superflat world.
Each type of biome has its own biome id shown in the following table.