Yep, seen this happen in a LOT of places, including areas where Nordheimers spawn. The area that’s most notorious for this is the region around Riversend (C9/10), but I’ve also seen it happen…
- inside the cliffs adjacent to the northern Sentinels statue (G/H4)
- on the north side of the ridge north of New Asagarth (F10)
-
east of Telith’s Island (H8)
- inside the cliffs along the river gorge east of the Southern Aqueduct by Bonepicker Camp (G7/8)
As far as I can tell, it seems like spot where the heightmaps used for the terrain extend under one another or inside of one of the cliff meshes are prone to such mishaps. This means it’s in no way limited to the aforementioned spots because many areas of the map are naturally constructed in this way. (i.e. things have to overlap, otherwise there would be gaps in the world to see or fall thru)
While it’s been a looooong time since I’ve done any map development in Unreal Engine (like nearly 15 years), my suspicion is that the heightmaps include layers that are “painted” to signify where thralls can and can’t path or spawn. If so, it might be an issue of adjusting the portions so painted so they don’t show up as being valid in spawn calculations.
Of course, if it was that simple, I suspect it would have been fixed by now, so I’m inclined to think it’s a matter of disparate complicated systems not playing well together. 
EDIT: Added a pic of the G7/8 location.