Got the same deal recently while out of uniform (business casual, slacks and a button up). Went to precheck line, told to use KCM, told them I can’t as I was travelling personally intl, allowed thru. Shortly after I was intercepted in the terminal by a couple of suits, escorted out and put into a general screening line behind the wheelchairs.

I’ve been told by my co and union that we are absolutely allowed to use precheck while in or out of uniform.