The icon had a question mark. It turned out to be 1 unit which was destroyed, but the province didn't fall. I am stiill in combat against non existent units which are destroying mine.
If that IS in fact what truly is happening, you need to report it and get it fixed. BUT, I suspect what is happening is it's a high level fort and reinforcements got there while you were asleep or otherwise occupied.
I have the same issue,and its shows there the fort lvl is lvl 1 only and it didnt show the enemies number though were already engage in combat and there is an instance that it allowed me to pass by enemy fort while were still in combat wich is impossible i think.