Currently the NPC Sensiria - NPC - Cataclysm Classic is phased for any pre-cata characters. Tickets have been raised to support about it but I don’t see it being fixed any time soon. An alternate solution would be to reduce the quest number requirements down to 43 for horde. This would resolve the NPC issue and award the achievement to those players who have completed the zone.
It is a problem not related to being pre-cata. I just finished the zone on my tauren that was pre cata. who is 85.