EDIT 2: I gotten the idea to use an external program to view my world's level.dat file and check what the biome ID is for this biome. It turns out this "Plains M" biome is actually the Roofed Forest M biome! This means this is now a bug report on Roofed Forest M biomes not generating trees or tallgrass. I just attached the pictures that shows the biome ID of the biome. Hopefully this makes squashing this bug easier!

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I do not know if this is intended or if this is a biome that was forgotten to be removed so I am making this bug report to find out.

Anyway, I have been exploring some seeds and I noticed that anytime a Roofed Forest is mutated, it creates a interesting new variation of the Plain biome. I'll call this new biome "Plains M Biome" from here on out to help keep things clear. Now what's interesting is there is no Plains M biome for Java Edition and it does not have any mutated form of the Roofed Forest Biome. Instead, it seems this mysterious mutated biome is exclusive to Bedrock only.

I also included pictures of Plains M biome that also have the seed and coordinates in them. After exploring several of these Plains M biome, I found some interesting characteristics of them:

• The grass color of this biome is a pure green color that matches a normal Forest and Roofed Forest's green grass color.
• No foliage will generate with the exception of patches of Dandelions. (no Tall Grass at all)
• Plains M's terrain is normally flat or hilly but can be as tall and mountainous as an Extreme Hills Biome.
• Plains M seems to be the mutated form of a Roofed Forest.

Now, I was able to deduce that it is Roofed Forests being mutated after seeing that all Plains M is always touching another mutated biome like Bryce Mesa or Sunflower Plains but is also always touching a Roofed Forest.

To test this theory, I even tweaked a Java Edition biome finder program to work with Bedrock biomes and set it out to find a Roofed Forest biome touching a mutated Extreme Hills. The seed that came back ,and that I tested, had the spot with the Roofed Forest completely replaced with the Plains M biome but the Extreme Hills M stayed the same next to it. Thus my theory was confirmed! One would think that the Plains M biome would be a mutated Plains biome and not a mutated Roofed Forest but apparently not! (The actual mutated form of Plains is always Sunflower Plains)

As you can see, the Plains M biome is very unique and all biome lists that I can find online never mentions this biome or any of its characteristics. That is why I made this bug report to see if this biome was intended to be generated or if it was simply forgotten to be removed for whatever reason. Thank you for you time! 

Edit: I double checked on Java Edition and it seems Java Edition has a Roofed Forest M biome that is the mutated form of a normal Roofed Forest biome. But in Bedrock Edition, no Roofed Forest M biome will generate and instead, this Plains M biome generates in its place. So this means Bedrock Edition is both missing a biome and has an extra new biome when compared to Java Edition. What is also possible is that the Plains M biome is actually a Roofed Forest M biome but it is glitched and is now missing all of its foliage and plants.