I had a slightly different issue, when I updated my iPhone 7 to iOS 14.5.1, the app Grocery IQ would not launch, iOS said it needed to be updated. When my wife upgraded her iPhone 12 to iOS 14.5.1 a few days later, Grocery IQ worked.
I offloaded the app on my phone and then reloaded it, and the app now works on my phone. My only guess is that there was some filesystem corruption in the app during the upgrade. Redownloading it by offloading and reinstalling fixed that.