Variant biomes / M-types / Sunflower Plains no longer being generated

Some biomes appear to be missing from the world generation. I've been comparing between the same world generated in 15w51b and 16w02a, and areas of the world generated in the latter snapshot have different (more common) biomes in place of those of the world generated in the former snapshot. For example, forest seems to be generating in place of flower forest. Also, I haven't (yet) found these less common biomes in 16w02a where they weren't being generated in 15w51b.

The affected biomes include flower forest and ice plains spikes, though there may be others.