It's even worse. I get the sign in alert on the phone home screen and in other apps as well at random times.
Did you ever find a solution to this problem? I am having the same issue where a sandbox testing account I created seems to be stuck in some corrupted state. I would think logging in to let the app store resolve whatever issue it keeps trying to complete (perhaps an unfinished transaction) would fix the issue, but unfortunately, I can't log in when prompted because the credintials can't be verified. I went as far as to delete the test user from iTunes Connect but that didn't seem to help.
Any ideas? Is there a way to "invalidate" or otherwise completely disable a previous sandbox testing account?