It's not erosion. I have a spot I leveled up and just had the top of a forge exposed so I could repair on it. Randomly the ground will cut away. Not completely and not always the same way. Seems stable now but always expect to have to do something,

Sometimes it even pops up more to bury the forge totally and I have to hit the area with a pickax to fix it. Seems to pop back with a single hit or level. Like that one area just isn't that stable on the map.