This is the very corner of a land-only map. For some reason, the mountains there have been extruded into a flat sheet above the ground. Yes, the ground is being drawn underneath it (and if we’re further down near the ground, it actually casts shadows on the ground).
Theory: this is probably an “off-by-one” issue, where the terrain is trying to share vertices between the north side of one block of terrain and the south side of the next, but the next block of terrain doesn’t exist (as this is the edge of the map), and so it’s wrapping back to the south side of the same block.
But it’s still a pretty odd-looking effect.
Pleasing that the shadows work with it, though!