Any idea of why this is happening?
If you not need navmesh on terrain gameobject you can uncheck on it “Navigation static” checkbox and do bake to see if this continues. If this not help and navmesh not on you road so you can also try experiment with parameters of bake navmesh. Try maybe enable checkbox “height mesh”, also try reduce Height inaccuracy %, and increase “Height” because it very small relative to “Radius”.