I try to enter Deadmines with my alliance alt lvl 18, but I can’t find DM in the list, kind of odd when I got the quest The Forman and it show as yellow.
Had to log into my horde character lvl 15 to see what level DM is this days in Classic Cataclysm, it show Deadmines 17-21, Ragefire Chasm 15-20.
So my guess it’s a glich or bug, just annoying when you try to do all quest in an area incl. Deadmines.