Implementing custom first login flow with custom idp mapper and custom user spi

Hi,
We have specific requirements to use our own user db that is we have custom user federation spi. Thus, for first broker login I need to synchronize and implement some logic in our user service. I’ve successfully implemented custom mapper which is getting called (I can see it while debugging through delegateUpdateBrokeredUser) every time when idp login attempts but importNewUser is never called.
In the documentation I can found that this method is

Called after UserModel is created for first time for this user. Called after “FirstBrokerLogin” flow.

My authentication flow is configured for allowing only existing user login and I need to interact with our user service when user logs in via Google/Microsoft for example. Maybe you can suggest more gentle way in spite of adding mapper to do the job.

Thank you.