Basic authentication fails with external AutoFill Password manager

Not sure how much Mull or Fennic etc can do about this, as it applies to all Firefox Android forks, but if you have a page that requires basic authentication that shows the option to autofill via a password manager, it fails and goes into an endless loading loop.

For reference, a Nextcloud password manager on Android where this error also occurs.

Using Firefox's built-in password manager works.

If this is not the right section, where would be the right place to post this issue?