I'm not too sure why it would be doing that. Mine works fines. I use the enemies and aggro enemies when weapon is drawn... especially now since I am leveling my gladiator.
Try to make sure you don't have any other filter settings checked. for example it targeting cycle function below the weapon drawn/sheathed option, and check your filter settings on the page just to the left of the filter tab... i think it is called targeting or filters.
Using the enemies only filter when tanking is vital because you can see which enemies are grouped together during a mob pull.