Equally, where would it be established that he wasn't aboard during the intervening time between STV and STVI?Where would it be established that Chekov stayed aboard beyond ST5 (an adventure that only involved a few days, during which the ship's systems were not functioning properly)?
To be sure, if he is shown to be a serving officer aboard the ship in the two consecutive films then the burden of proof is not on proving he was aboard for the intervening years, but rather on providing evidence that he was not onboard for those years if one feels otherwise.
In the same way that I don't feel that evidence is required to establish that Commander Riker was a serving officer aboard the Enterprise between First Contact and Insurrection, to be sure.