Out of frustration with this exact issue, I put my terrain back to the original import height and the Landscape Grass Type started populating (total guess work as I had not seen this thread) I then put it back to my intended Z location and the grass went with it. Not sure if you have editable layers enabled along with world partitioning — I do and I am not sure if this has any relevance to the bug. However, It appears to be working fine at the moment.