Based on current tradition then, Spock shouldn't have been called "Mister" at all and yet he always was.
Still seems odd to me, when the rest of the crew called him "Mister Spock", especially lower ranked officers like Chekov and Kyle and Sulu. Hell, everybody
called him "Mister Spock".
To have an ensign call a commander (XO no less) "Mister" seems inappropriate.