I was having this exact same problem here:
http://community.auth0.com/t/using-sample-php-code-on-ios-12-auth0-login-page-hangs
This was using any mobile iOS browser - Chrome or Safari on all iOS devices for 50% of my iOS logins.
It eventually “just stopped” happening so I was no longer able to recreate to show the Auth0 team, but there is a real problem here on the Auth0 side.
EDIT TO ADD: Try using the “Universal Login” instead of the classic login. That always works.