Thanks for getting in touch with us at Auth0 community.
This is a known limitation at this time in regards to using a custom database with import mode off and performing a password reset on a custom DB using a username. The change_password script only handles an email.
We have an open feature / bug report to add the username field to the change_password script. Unfortunately I don’t have a firm ETA yet on when this will be implemented as it is still in backlog.
I would suggest you add some feedback about this here https://auth0.com/feedback as this goes to our product teams and it will help gauge demand for a feature.