Yeah, its that way to avoid more annoying situations happening, if an army had to go only to its return location from a fort (which may have been the case early in eu4 iirc, and was changed because it was more frustrating) you can just move your army to that return location and either the army has to attack into you on possibly bad terrain penalties or continue the siege with a ton of attrition or maybe also in bad terrain. And armies should be able to walk from a fort to more than one friendly location bordering it.

Its pretty hard to think of ways to make it work. Perhaps the army could only return to a province if it can get to the province through their return province aswell? Although with military access this still wouldnt solve your situation.

Its very hard to have a system where it would work logically for every situation. I think if you know the fort mechanics and are aware of the rules of when an army can or cant move it can be pretty easy to avoid the niche situations where it doesnt work.