[Edi]

That's the army movement bug, where you move an army to attack but it doesn't. But since you're in a province where you siege, instead of the defend province command, the commanders defalut to maintain siege.

Send the files if you want, but I don't know if it will help at all. It's not been reliably repeatable before, which is required for it to be solved unless it is discovered while fixing something else. See recent spate of fixed immortality bugs, they all have different times of addition to the list, but seem to have been linked and were taken care of at once.