I've checked with our developers on this question - the User field in the "Operating System" section displays the name of a user that is currently logged on. This property must have a value. In a terminal services session, UserName returns the name of the user that is logged on to the console not the user logged on during the terminal service session.
Of course, I've checked this with our development department, so this is the information I got directly from the developers. For more information please also try referring to this Microsoft Documentation page.