While using Graph API (Beta) 'credentialUserRegistrationDetails' does not include the 'Default' registration method, something that can be done using PowerShell. It will be useful to track what's the preferred registration method across the organization. Another simple workaround would be return the authMethods property in order, where 1st is the default. Thanks!
Microsoft doc: https://docs.microsoft.com/en-us/graph/api/resources/credentialuserregistrationdetails?view=graph-rest-beta&preserve-view=true
