Specifically in dev version 2.3.1 all default mobs spawn in "mushroomisland", "mushroomislandshore", "hell", and I assume "sky".
Also, any attempted custom spawn list for biomes get deleted (this is with custom biomes/not default, and the biomes do not have the ID of a default biome.)

To test the second simply grab the ocelot example and attempt to get them to spawn in any biome, the line will be deleted and reverted back to normal.

I added mob spawning back in 2.3.3. I think that you were using an unofficial 2.3.1 build (mob spawning was disabled for some time).