Based on current tradition then, Spock shouldn't have been called "Mister" at all and yet he always was.
Technically yes, but Spock's rank was kind of squishy for a while. Plus it had been "Mister Spock" clear back to the early drafts when he was a lieutenant, so it probably never occurred to anyone.
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.
Well, maybe Starfleet's tradition was that all officers but the captain could be called "mister." If it's appropriate to address a lieutenant commander that way, being incorrect for the next grade up seems kind of arbitrary.