Although in the VOY episode "Flashback" (concurrent with The Undiscovered Country
), Rand has the rank of Commander or Lt. Commander, from what I remember.
Basically, the whole Rand rank-issue is another discontinuity from that episode (alongside Lt. Valtane "dying," then showing up perfectly alive and well on the viewscreen at the end of the movie).
Imperfections in Tuvok's memory-engrams from the events in that storyline, maybe?