From the ATProto docs:
When passing an account identifier through to the Authorization Server as part of the Authorization Request in the
login_hint, it is recommended to use the exact account identifier supplied by the user (handle or DID) to ensure any sign-in flow is consistent (users might not recognize their own account DID).
This is also mentioned in issue #9 - support passing handles as login_hint to oauth server.
The login_hint value is already available where necessary, it just needed to be used instead of the DID.