When was any Starfleet Admiral referred to as a "flag officer?"
I don't believe Starfleet uses that term.
Como. George Stocker, "The Deadly Years": "Mr. Spock, I do not like what I'm about to say, but regulations demand it. And as a Starfleet flag officer, I must follow regulations."
Lt. Saavik, The Wrath of Khan
: "General Order Fifteen, 'No flag officer shall beam into a hazardous area without armed escort.'"