Screenshots please. And some wizard will explain it according to fort rules. We get 2 or 3 of these posts a week and afaik not a single one of them ended up with breaking forr zoc rules
I've done this 3 times in the past with esoteric edge cases and the "wizards" failed all 3 times.
Anyway, the OP's particular scenario as described is known/intended behavior consistent with the wiki and is just a bad design interaction. Capturing a fort can reduce your movement options because you get a new return province. Despite that ordinarily a hostile fort would limit movement more than a friendly one.
AFAIK you can still return province swap them out of it, which showcases one of the core issues with EU 4 ZoC (two armies on the same province fighting the same battles for the same nation have different movement rules, by design).
There is just so many fucking loophole and expectations that no sane human can keep track of them anymore.
Well, you guys fixed one of them not TOO long ago (maybe a year or two now?). It used to be possible to deliberately exile troops, then queue movements in a way that would bypass ZoC after clearing exile. I'm pretty sure that wasn't intended! Nowadays the game will interrupt that rather than let troops calmly march through infinite ZoC via continuing exile orders.
Assaulting forts is still bugged today. The game claims only X number of people can assault. But there is no cap on casualties; clearly > 10k are participating in the assault. You therefore get WILDLY different outcomes if you just order an assault...versus if you order a fraction of the army to move and regroup/merge army on each battle tick to keep ~max soldiers assaulting (and not much more). In one case, you get a failed assault and massive casualties. In the other, you get a consistently successful assault with still-significant but much lesser casualties. The only difference is some extremely painstaking swap/merge micro. Is that REALLY intended, and not a bugged interaction with assaults?
If only 10k can assault, either that's true or it isn't. I don't see any design benefit to forcing tedious micromanagement of army swapping/merging in the middle of assaults to get a better outcome.
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Also, you can still mess with return provinces via army swapping, and there is still a bug that associates movement lock with the presence of a fort (causing same or adjacent province shattered retreats even in regions of the world with all forts deleted). If you mouse over it will even say "land movement blocked by hostile fort"...in your own territory with literally no forts anywhere in 5-10 province radius. Not easy to reproduce, but it's still happening as of current hotfix. This is sort-of a fort bug? One the one hand, there is literally no fort. On the other, the game seems to think there is?
I also have an old screenshot lying around where I couldn't queue a unit to return to the province it was presently in because returning to the province it was already in was blocked by a hostile fort (???). This one resolves if you stop the unit and re-order it, but can be annoying if you're in MP/not wanting to pause. I was never able to pin down the steps for this one, but I run into it from time to time. Rarer than the above.
Forts are just a disaster. It's sort of funny that six years on were still have this incomprehensible nonsense in the game. Something as fundamental as this should be easy to understand at a glance. The whole concept just needs to be scrapped imo
Best suggestion I've seen is still to make ZoC slow units rather than hard block them. If you slow enough then big armies can't catch smaller ones w/o sieging the fort, and there are no meaningful tricks with overlapping ZoC, capturing forts or army swapping. For hostile/friendly forts both influencing one province you could either make them offset or have it slow both sides, either would work. Also solves the issue of "what happens when two armies enter from different origin provinces and merge", because it wouldn't matter anymore.
But it would be a big gameplay change and would shake up reactions a lot, again. So I can see why they might not want to do something like this again in EU 4. Still, ZoC remains a mess right now, and somewhat at odds with how the game's movement/threats of armies/positioning were originally designed.
The bug with assaulting forts is bad too. It takes a LOT of micro to avoid it, and assaulting forts is otherwise a legit way to punish armies being far out of position.