"Remember my username" causing redirect loop, then error on iOS 14 Safari

Bug description

Link to customer issue: https://forgerock.zendesk.com/agent/tickets/54239

On iOS 14 or above when XUI’s “Remember my username” is checked, after successful login, if the user then logs out and goes back to the login screen it goes into an infinite loop. You can simply test it with http://auth.forgerock.com on iOS 14 (so far I confirmed that this happens on both Chrome, and Safari app). It does not happen on Mac or other platform. And attached screenshot it’s the network log when the loop happened. Does anyone have any idea on what’s causing this issue? And again, this only happens on iPhone devices, and simulator with iOS 14

How to reproduce the issue

1. Go to http://auth.forgerock.com login and check the "Remember my username" checkbox.   
2. Sign out again and it gets stuck in an redirect loop until finally the text "A problem repeatedly occurred..." is displayed.

Expected behaviour

Login page is displayed

Current behaviour

Redirect loop, then error message

Work around

Don't use remember my username?

Code analysis

The "remember my username" checkbox sets a cookie. When you go to the login page, it checks for this cookie and if set, it loads the users username from the cookie and then tries to autofocus on the password field. On iOS autofocussing on a field brings up the onscreen keyboard automatically and apple has tried to block this as it can be annoying for users.