From the Safeguard Administration Guide:
All users part of a directory import user group must have complete and valid attributes. If
the attributes for a user are not complete and valid, the user is not imported and the
import continues. For example, if you set the directory user group authentication
properties to require secondary authentication and use the Starling 2FA provider, each
user's email address and mobile phone number attributes must have values to be included
during the import."
Ensure the user has the mobile number populated in the correct format as well as an email address configured.