Ok, I tried some recommended fixes I found online such as building small plaza’s around it or playing around with roads, none of which worked. Also, just flattening over it several times didn’t work. What did the trick, though, was really leveling down the terrain a bit with the flattening tool and then leveling it up again. Probably would also work the other way around. However, I was only able to do it because I had terrain with different heights around, as you can see on the screenshot. So, if you have this bug on flat ground (which I already had several times in the past), you probably will not be able to fix it…