I don't know who mentioned it, but when you place a road over a mountain, and then remove the road, there is grass underneath.
But I think in the first place, we aren't meant to be able to build roads on mountains?
I can confirm the same thing happens to the coastline, removing the road turns it to grass
Roads need a little rework, currently placing a road erases whatever was beneath it, hence why everything turns to grass.
Not building roads on mountains makes sense, maybe with a different type of work designation that could flatten the mountain (to make a pass, or allow for building a mine)