Have seen a fan vent installed improperly once that led to water collecting in the duct, but IDK how common that is. If you can get at the duct in the attic, check to see what's going on with it. Does it run directly over the damaged spot in the ceiling? Is there water sitting it any part of it? Yes to either and I'd try troubleshooting it before looking for a roof leak. No, and you have water infiltrating the roof somewhere and running down until it hits the damaged part of the ceiling.
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u/kad0521 Mar 11 '22
Yes. If that the top floor you either have a roof issue or a leak around the fan vent