Notification badge stuck on app icon

I have a very strange issue with an app on my iPhone 14 Pro (IOS 17.5.1).

The app is allowed all kinds of notifications and usually works correctly like any other app; however, some times ago (I think a couple of weeks), I received a notification and after reading and deleting it the red badge with "1" on the app icon got stuck; the app icon always appears as having one pending notification, however there are no notifications anywhere, even inside the app itself (which has its own internal list of notifications).

What I tried:

• Force closing the app; nothing changes.
• Rebooting the iPhone; nothing changes.
• Disabling badge notifications (or notifications altogether) for the app; the badge disappears, but when I re-enable notifications it reappears.
• Last but not least (and this is what I find most strage), uninstalling the app. But then, when I reinstalled it, for some unknowable reason its icon appeared again with the red "1" badge.

I would consider this to be a bug in the app, but how can it survive after uninstalling and reinstalling the whole app?

I can confirm all application data is deleted when I remove the app, because after reinstalling it I have to go again through its first-run setup (during which the app asks for consent to show notifications and you have to explicitly grant it); however, it looks like the phantom pending notification remains stored somewhere on the iPhone, because immediately after reinstalling the app it is already present on its icon.

Also, during this time period I received other notifications from the app; but the number on the red badge always remains "1".

Any idea about how to fix this?