I noticed that some of the birch forest biomes changed going from 1.8 to 1.9. It looks like all Birch Forest M biomes from 1.8 are now generated as Birch Forest Hills M, while all Birch Forest Hills M biomes are now simply Birch Forest Hills . I included a screenshot for comparison (unmodified clients, fresh worlds).

At this point I wasn't sure if it's a bug or an intended change. But the source code of the new biome definition list reads something like this (as far as I could understand):

registerBiome(155, "mutated_birch_forest", ... (new BiomeConfig()).setBaseBiome("birch_forest") ... )
registerBiome(156, "mutated_birch_forest_hills", ... (new BiomeConfig()).setBaseBiome("birch_forest") ... )

Note that both mutated biomes have their base biomes set to Birch Forest. In all other cases, the base biomes are unique and reference the biome whose ID is exactly 128 less (consistent with before). So this looks like an oversight to me and should probably read "birch_forest_hills" for the second biome.

As far as I can tell, the consequence of this is that the second biome replaces the first one as the mutation of Birch Forest. Whereas before the mutation mapping for the biome generator looked like:

• Birch Forest (27) -> Birch Forest M (155)
• Birch Forest Hills (28) -> Birch Forest Hills M (156)

In 1.9 it looks like:

• Birch Forest (27) -> Birch Forest Hills M (156)
• Birch Forest Hills (28) -> No mutation (stays Birch Forest Hills)