Most U.S. airline FOM's explicitly forbid drinking alcohol or even being in a bar while in uniform. Some international pax lounges will not let you enter in uniform even if you are on a ticket and from another carrier.
Same in Central Europe, even dead-heading, whatever.
Funniest thing I remember is what happened to a good friend, a naval aviator and then Defence Attache with a Central European nation to a northern African state.
He was travelling business class on duty in his finest uniform ... and was refused entry to the lounge as they confused/mistook him for a dead heading company crewmember. All his protests were futile.
Only later did the airline apologize after the embassy/foreign service concerned wrote a complaint and threatened to make it public.