something strange happened last turn: I took Jotun's fort and fought a "break-siege" battle there too as Jotun AI tried to attack my sieging force- nothing strange there.

But at the end of my turn (the last message)I was informed that my "undefended province in jotunheim has been taken by the enemy"

I checked and somehow barbarians managed to attack me AFTER I took the fort...strange.

Anyway, this turn I broke that "siege" and am ordering destruction of the fort. I have last-turns file saved if anyone wants that confirmed independently.

I still don't know how that happened though.