In my case testers were added first as external testers and later as internal testers. This is what Apple Support send me: If the tester is listed as an internal and external tester under the same name or email address. The tester should: 1 Press the Stop Testing button in the TestFlight App, if visible. 2 Delete and reinstall the TestFlight app. You should: 1 Remove the duplicate user from internal and external testers. 2 Resend the invitation as an internal or external tester, but not both. If this doesnt work then pleas provide the following information, To better assist you, ....

Still here with same problem.