On every character so far that i have leveled, using the Dungeon Finder will show a list of available dungeons that will be chosen.
Each character upon hitting level 15, has the choice of either Ragefire Chasm or The Deadmines.
Each Character, upon entering a dungeon (and this could just be my own bad luck) has had Ragefire and never Deadmines. By the time a character hits level 17, Deadmines disappears from that Dungeon Finder List and another Dungeon is in its place. However Ragefire Chasm remains.
Too my memories knowledge Ragefire Chasm will remain all the way until a character hits level 20, upon hitting 20 or 21 only then will Ragefire Chasm disappear.
This is obviously a bug of some sort.
My suggestion … fix it or make the Dungeon list like Retails Dungeon List where every Dungeon is listed and only blocked out from selection due to level requirement and will be blocked out once you have gone way pass that dungeons level limit (2 or 3 levels higher)