Agreed.
I was in the military in 1970 when the movie was released and saw it at the base movie theater. All movies at the base theater began with the US Flag projected on screen and a recording of the National Anthem. All military personnel were expected to come to attention and salute the flag while the anthem played.
My guess is the projectionist wanted to play a joke on the audience that day and do something a little different. Some may remember "Patton" began with a full-screen picture of the US flag, followed by someone calling "Attention!" then Patton walking up from behind the "stage" on unseen steps to appear in front of the flag to salute while "To the Colors" played. He then told the audience "Be Seated" and gave his “No dumb bastard ever won a war by going out and dying for his country. He won it by making some other dumb bastard die for his country” speech.
When the flag appeared the entire theater stood at attention, saluted and spent a confused couple of minutes watching Gen Patton onscreen while "To the Colors", not the National Anthem played and wondering what the heck was happening. Only when he gave the "Be Seated" command did everyone drop their salute and sit. It was actually quite funny.