I'm experiencing the same issue now. The app was validated and uploaded successfully to the App Store connect and we got the notification, which also is visible on the test flight. But we can't install it. Its like the test flight app doesn't detect that the app is installed. Also the cloud icon is appeared to the app name and when I try to run the app it says: "The app could not be installed because its integrity could not be verified". Apple is so reliable that when something stops working everyone freaks out :D

I am facing the same issue.