The old Mountains biome, also known as Extreme Hills, only generates in tiny patches now, with only a few examples being big enough to be noteworthy.

During the experimental snapshots Henrik mentioned making them more "extreme," and thus they now generate in more extreme terrain like shattered savannas. However 90% of the time the shattered savannas completely take over along with other normal biomes, leaving the old mountains/extreme hills in small micro patches. 

Note that most old mountain/extreme hills are like this, it isn't a rarity like normal microbiomes. This creates patches off blue cold grass and snow among usually other non-snowy biomes. 

What I expected to happen:
Extreme hills/Old mountains to generate with the size of other biomes.