Depends if Mission Hub is using G.E. data which sometimes has a slight horizontal shift where the altitude may be in error and jump greatly. Odd altitude jumps sometime happen just moving a mouse pointer on a G.E. map.
In the above GIF, which generates 4 waypoints off the KML (i.e. The four vertical red bars in the green area), the third one at the cliff's edge puts you inside the mountain where the 4th is correct on top of the mountain. The terrain is slightly shifted when it goes through the calculation process for the mission verses what it draws for some bizarre reason.
I'd like to see the test flight icon reporting the altitude since we cannot really put the mission horizontal and see the z-axis or vertical height from the flight mission making. Might show things like trees and building clearances better too. A sharp cliff wall is another matter since the G.E. map should be accurate. Too bad these mission maps are only in 2D where we fly in 3D. Probably better off dropping your own waypoints and checking than using a KML generated G.E. file to do so.