When inserting a vertex on a terrain using the edit tools, the vertex is put at 0 elevation (if outside the terrain) and at the terrain elevation (if inside the terrain) even if you snap to a 3D element. The current workaround requires moving the point to the 3D element after the vertex is inserted to get the correct elevation. When inserting vertices, we usually have 3D elements to snap to, and we want the new vertex to get the elevation from that element.