The issue with the grass is a known feature/bug of the scenery tools. The thing is our airports came mainly from the X-Plane community, and they define transparent layers in a way different from our scenery tools. It is a bug in this context, but there is not anything wrong in the way we defined transparent layers. In fact, there are ways to make these layers compatible with X-Plane and FlightGear (make them lines instead of layers), but people making airports for X-Plane do not bother about compatibility with FligthGear.
This issue affected very few airports around the world and, since our human resources are scarce, it was not a top priority. It is funny how this affects now our default airport so it suddenly has become a top priority issue
I prefer a once-and-for-all fix regarding transparent layers, but a very quick fix is just ignoring transparent layers in the apt.dat parser (optional manual intervention: remove these lines) and regenerating the airport.