On earlier FG version I only ever seen the black grass when I had compositor active ( also some other missing terrain textures ) but in the 2020.4 version im on at the moment, thats not happening now.
@Stuart, I didn't think Compositor was even available yet on MacOS. I haven't found a menu option for it. ludomotico said, on the .org the other day, that " I noticed the compositor seems to be activated by default in the last nightly: buildings have shadows." Maybe these are connected?