Problem: Grass clearing is wrong and not accurate. At some places it may be okay while at other places it's bad. There may be more clearing on one side of the path, while not clearing far enough on the other side at places.
Background: This can happen if the Detail Resolution of the terrain (in world space) is too low for the kind of work you are trying to do.
Solution: You can either increase the Detail Resolution of the terrain, or make the terrain smaller, so the same resolution applies to a smaller space. This will increase the terrain's world space resolution and provide better results. You can find these settings under the Terrain Properties tab of the terrain: