Glad you found a solution to your problem. I had a problem awhile back with a small section of terrain. After levelling & flattening a strip of ground at the entrance to one of my buildings, when I logged off then logged back on strange things began to happen ...
https://steamcommunity.com/sharedfiles/filedetails/?id=2687397099
https://steamcommunity.com/sharedfiles/filedetails/?id=2687398168
https://steamcommunity.com/sharedfiles/filedetails/?id=2687398370
https://steamcommunity.com/sharedfiles/filedetails/?id=2687398600
https://steamcommunity.com/sharedfiles/filedetails/?id=2687398881

Once I had it levelled and it stayed nice and even for 3-4 days, until I grassed it over, which is the last screen shot. I levelled it out and it seems to be staying, so I am taking no chances, "No grass for you!"