The Federation ID field on the new user records is not correctly set is the probable cause of this behavior. The Federation ID is an additional field contained in the Salesforce interface that allows admins to pick whatever username or username format they want to pass to Salesforce from their user directory for single sign-on. This field does not appear on the user pagelayout editor or on the user record page by default, and it must be populated with a unique value that matches the identity provider’s assertion for each user. If the Federation ID is missing or incorrect, the SSO will fail. The administrator does not needto reset the new user’s Salesforce password, as SSO bypasses the password authentication. The My Domain capability is not enabled on the new user’s profile, but on the org level, so it does not affect individual users. The new users do not have the SSO permission enabled on their profiles is not a valid option, as there is no such permission in Salesforce.