So, after some investigation, it appears that the 'dirtgrass' tiles are not in the appearing in the tileset.
It's there in the image, but not coming up in the set and I can figure out how to fix it.
My temporary fix was removing all the 'dirtgrass' lines out of the Rules.txt and Blends.txt.
So, I'm leaving this open as unanswered, in case this causes an issue down the road.