Yes, you can not pass away from their ZoC deeper into the country before sieging down the fort. This is intended behaviour according to the design.
Is it also intended that you can't move into your own territory, away from a fort? After seeing this, I realized that if I timed a DoW I could literally trap armies from moving in their own territory using forts. While this turned out bad for me, understanding its nature suggests to me that I can time-DoW to abuse the hell out of it, especially against the AI:
I can't see how this functionality is viable. Knowing this, I could literally wait until an AI army is nearby, trap its armies, siege its capital/carpet non-forts, and it wouldn't be able to stop me until it gets rid of the fort. It can similarly be used to completely hose/block pieces of an alliance while you wipe/kick allies out of a war. If this is WAD, I don't think it should be, you can do some pretty broken actions with it.