I wonder if this is related to the issue with the Enemy of Justice condition reported at https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=879196. Or at least, a consequence of the same underlying problem with the way the game seems to be handling certain NPCs/guards.

I completely agree that this shouldn't be oath-breaking. If you haven't already reported the bug to Larian (the Report a Bug link at the top of the launcher will take you to the right place), then that seems worth doing.

"You may call it 'nonsense' if you like, but I've heard nonsense, compared with which that would be as sensible as a dictionary!"