Upon creating several dimensions (with different terrain types only), it appears that some terrain types do not want to spawn grass. They instead spawn dirt, even with foliage on them. This isn't a huge bug, but it is annoying since there is no grass material modifier.

Screenshots:
Nearlands WITH grass: http://i.imgur.com/nDYNExO.png
Chaotic WITHOUT grass: http://i.imgur.com/glDbKbU.png

This most likely has to do with the biome.

Closing due to lack of other information. Reopen if you have some more information like biomes and such.