Biome height generation is messed up in custom dimensions

Ok, this one's a lil complicated.

Basically, if I was to make a really mountainous biome, and a really flat biome and added them to a custom dimension, do you know what the terrain would be like?

Rather than the height being according to the biome, the dimension would be mostly flat with a huge mountain every now and then positioned randomly, regardless of the biome it spawns in. The same happens setting a biome with a negative base height - other biomes go over the water.