I had this posted in the general forum, since I didn't think it was a bug, but now I'm thinking it might be. While I can't reproduce it, I do have the saved games from both before and after. They're about an hour apart (real time), unfortunately.
Anyhow, there's this white box next to Durance, like he has an affliction. You can see the effect in the 2nd image:
I've camped twice and rested at an inn once with zero change to Durance's status. His health and endurance are at max. At an inn, I swapped Durance with another NPC and the white box went away, seemingly fixing the issue.
Finally, here are the saved games:
Before affliction:
https://www.dropbox.com/s/rimt3gm0yzadvp6/82c73861fd2444259c74a291ec9de281%2012753423%20GreatHall.savegame?dl=0
After affliction:
https://www.dropbox.com/s/a677h1nqg7ax2zb/82c73861fd2444259c74a291ec9de281%2012879714%20MagransFork.savegame?dl=0
UPDATE: I've edited the post--deleting the irrelevant parts--because I originally blamed some of Durance's stats on the white box bug, but found out that the stats were unaffected. To the best of my knowledge, the post is now 100% accurate.