This appears to be a direct result of the texture streaming pool being too small. In the console, enter r.streaming.poolsize xxxx, where ‘xxxx’ is a number big enough to handle your texture load. I needed 4000 to handle the very large texture pool my project requires.
Odd that there isn’t a more explicit warning message. It took me forever to figure out what was really causing grass maps to stop building. A simple “No available texture pool for grass maps” would have helped a ton.