In the latest Bedrock edition, Birch Forest M (the biome with unusually tall Birch Trees) will not generate. To make sure this is the case, I found Birch Forest M seeds for Minecraft Pocket Edition before Bedrock Edition was released and tried those seeds on Bedrock.

The seed I tried was 1560093373 and the Birch Forest M biome is suppose to be directly in front of spawn. Except, when I try the seed, I get just a normal Birch Forest and not the M variant. Other M variant biomes may not be generating as well since I do not recall seeing a Plain M or Swamp M biome either when I am seed hunting.

In the pictures attached, one picture shows the mountain with the Birch Forest M and the other picture is my screenshot of the normal Birch Forest I see in my game where the M variant is suppose to be.

EDIT: The other M variant biomes seem to generate normally including Swamp M biome. However, it seems Plain M biomes are turned into Roofed Forest M biomes. That's a shame cause Plain M biomes were cool to find but I know this was deliberate. The Birch Forest M biome not generating is definitely a bug and will need to be patched.

Edit 2: Another seed to try is -1114341188 and you will spawn with a birch Forest in front of you. This Birch Forest is suppose to be a Birch Forest M and it used to be a Birch Forest M in previous versions of Pocket Edition. I attached two more pictures (IMG_6803.jpg and IMG_6801.jpg) showing the old version and the current version.

Edit 3: Added 3 new pictures of a third example to prove the M variants did not change locations at all. The seed is -1879528770 and had a Savanna M/Birch Forest M combo to the left of spawn in the past. Now, the Birch Forest M is reduced to a regular Birch Forest but the Savanna M is still a Savanna M. In addition, Birch Forest M are a more mountainous variant of a regular Birch Forest. With that in mind, the huge mountains on the left in the old picture is generated only because it is in a Birch Forest M. Now, the mountains are still there but the trees are no longer tall. This means the game knows this exact spot is suppose to spawn a Birch Forest M as it does with the terrain but is spawning a regular Birch Forest as the biome instead.

Edit 4: Still an issue on 1.2.9. I included InsaneBirchForestM.png picture to show that the biome is definitely a Birch Forest M as the crazy terrain shows it is far too mountainous to be a normal Birch Forest but the trees are still bugged to be regular height.  The seed for this Birch Forest M is 1365449400 and the coordinates is -30 77 250.

Edit: Still not fixed in 1.8. 1.9.0