Noticed this as well on single player, no mods. It’s like the AI just stops functioning at high altitude, so I figured it was related to all the “can’t place thrall” stuff folks have been mentioning.
Enemies stop working too. Rocknoses, dragons and giants have all just stood there and let me chop em up in those really high locations.
I see a similar issue daily on the official server I’m on, and its not just high altitude mobs. Its all fixed after restart, something happens over the course of the day that breaks them.