Active Directory User Cannot Login When UserPrincipalName (UPN) is Different Than Default Realm When Using SSSD's AD Provider

Issue

Active Directory users have a UserPrincipalName attribute within Active Directory. According to Microsoft Documenation:

This attribute contains the UPN that is an Internet-style login name for a user based on the Internet standard RFC 822. The UPN is shorter than the distinguished name and easier to remember. By convention, this should map to the user email name. The value set for this attribute is equal to the length of the user's ID and the domain name.

If an Active Directory user has an "enterprise principal" other than that of the default realm, sssd will fail to log the user into a Red Hat Enterprise Linux 6.4+ system leveraging the ad provider.