Get-ADUserResultantPasswordPolicy return nothing for users use Password Settings from Default Domain Policy

While testing Get-ADUserResultantPasswordPolicy, I notice it returns nothing for users that are use nothing but the Password Settings fromDefault Domain Policy. But if the user has another password policy (e.g. Password Policy for admin accounts) set to it, the command does return correct password policy. 

What could be the cause of this behavior? 

Is there another command I can use for both kind of users to get their password expiry date?

Thanks.