There is currently a PR created upstream where there is some discussion on whether or not using preferred_username to resolve an user entity is ideal/safe or not.

This requires more investigations on the common OIDC providers used and how each provider handles preferred_username creation/change since each provider handles this claim differently. Depending on how easily users can change this field, it may or may not be a safe resolver to use/contribute to upstream. 

The ideal solution to this would be to go with this approach, but it would take much more time and testing.