Tab should only cycle through mobs in line of sight, and in front of the person. This seems to be a new problem, I never had it before... Maybe some walls and obstacles in these new dungeons do not register line of sight properly? This happened to me in Lost City and Tol Barad.
Another example of this issue is trying to LoS a pack of casters behind a fountain/wall in Lost City. Sometimes casters will just shoot at you through the obstacle, and not move an inch.