As per our documentation, I do not believe tableau takes only the UPN Prefix :
userPrincipalName(UPN). This attribute specifies a user name in the format
jsmithis the UPN prefix and
@example.comis the UPN suffix.
In the Windows Active Directory Users and Computers administrative console, the UPN is a concatenation of two fields on the Account tab of the user object: the User logon name field, and the domain drop-down list next to it.