If one team doesn't finish a map, but completes one of the objectives, and the other team finishes that objective faster, the game doesn't end, even though victory is already guaranteed. It will end if the other team completes an extra objective automatically, but not if that team completes the same objective faster.
I just got finished with a game where both teams couldn't finish the last objective on train. My team went second and manage to finish the first two objectives faster, but ran out of time on the last. When the game ended, we still won, but the game should have ended when it was impossible for us to lose.