TestFlight App does not open when View in TestFlight button is tapped

I have one tester who seems ot have a problem using TestFlight. For all other testers, when the user taps the "View in Testflight" button in the email invite, the TF app opens. But for one user, when she taps the View in TF button, Safari opens instead and displays the redeem code instructions for redeeming the code (which does work, but it is time consuming, cumbersome and she should get the same flow everyone else gets) - why is this happening?


She has deleted the TF app and re-installed it (to make sure she has the most recent version) and I have re-sent the TF invite 3 times, each time with the same result.


Why is this happening to this one user?