I had a very similar problems with MiniHack app on my older iPhone, that it stopped working after iOS 14.5 upgrade, but which still worked on a newer iPhone 11 that also runs 14.5.

Came to these comments specifically to find a reason for that (reinstalling an app was not an option because I think the app no longer is visible in the App Store).

Thanks to @xuki, I tried offloading and reinstalling the app, and it fixed the problem!