...
After they click allow - they need to:
refresh the site again
...
try to login again
at which point they should be able to succeed.to successfully login.
Future logins won’t prompt them to redo this for some time. I believe Safari requires the user to have direct access to the original site every 30 days. (This is what the popup “My Authorization” page does for them.
Testing Safari - Reset
If you’ve already tested this process on an iPhone and want to reset it to try again - you have to Clear History.
Settings
Apps
Safari
Clear History and Website Data