When using the custom world types Caves or Floating Islands, there is an issue where, in the case of Caves, netherrack/lava lakes generate either instead of or as well as the blocks related to the actual biome chosen, and in the case of Floating Islands, endstone generates along with the blocks related to the biome chosen.

I haven't tested it with all biome options yet, but so far the bug has occurred in every single one I have chosen, which is Birch Forest, Flower Forest, Taiga, and Badlands.

So far it looks like only blocks are affected, while mob spawning is how it should be (Overworld mobs in an Overworld biome, etc.).

Attached are screenshots of the Caves world type with chosen biome Badlands, and Floating Islands world type with chosen biome Birch Forest.