Autofill previously saved password on a new domain inside ASWebAuthenticationSession

Is there some way to suggest to the system that it should autofill passwords in a ASWebAuthenticationSession on a domain not previously associated with my app?

I'm working on an app that currently has native login and registration screens, password autofill works great and links credentials it to my associated domains.

I now need to move these login and registration screens to use ASWebAuthenticationSession and have users authenticate on a website on a new domain. The system does not autofill my apps credentials on these pages.

I can add this new domain as an associated domain, and new registrations are able to save the password and then autofill it later. But I can't get previously saved passwords to autofill on this new domain. Is that possible?

I did manage to use Shared Web Credentials to save the users credentials against the new domain when they login. But this will only migrate users that login between now and when I switch to ASWebAuthenticationSession, not all users will be migrated.

Any new ideas or solutions much appreciated.