After manually resolving a stronghold attack (skeletons attacking the keep), the screen did not fade to black and reset as usual. Now whenever I enter the main keep, the skeletons have respawned (and the other NPCs such as the general goods vendor do not spawn). I have killed them all several times but every time I leave and re-enter, they come back. As far as the stronghold screen is concerned, the attack has been defeated.
I would have attached my save game for further investigation, as per the rules, but apparently the attachment limit is too low to allow me to do that. Perhaps that deserves a separate bug report?