Trying to Understand Forts (Illustrated)

[LittleFido]

I have not been able of reproducing this particular event. (Does seem like a bug to me at first glance).
Generally Forts would stop you from advancing deeper into enemy territory, however they would not block you from going back toward your own country, unless there is another Fort on the way back that would block your path. In which case this fort would block you from being able of returning to your home country. (If you let your enemy disrupt your lines that is your own fault)
You would not happen to have a save available?

 

[Dnote]

ZOC is only on the same controlled territory as far as I can tell, but I have noticed odd instances where the AI has been able to move through a fort zone without issues.

I think it would be better if ZOC applied to all allied provinces as well, not just the same nation as the fort belongs to. I find it a little strange that I can go through a fort in Switzerland into their allies territory, but can't move through the fort to another Swiss province, even though I'm at war with both of them.

 

[LittleFido]

I did poke around a bit in the region and did observe some rather odd behavior when it comes to blocking movement.
However, a deeper investigation is needed before we can tell with any certainty exactly what the problem is.