So far Space Privacy is not fully compatible with Confluence Questions, meaning the privacy features may not work as expected. A question of an extranet user can be seen by all other users (including the authors name).
We recommend to restrict the Confluence Question usage only to internal groups (the ones you might have configured in the space privacy configuration).
See the Confluence Questions documentation for further details: Permissions
This will exclude extranet users from Confluence Questions, but ensures privacy.