Can’t pick any dungeon besides Dragonguard Depths (the background even changes if you try to do so, but stays on Dragonguard Depths; EDIT: it’s a graphic glitch).
However, even once a dungeon is picked, the game snaps that dungeon right back to Difficulty X.
That would explain your level 100 chest.
In any case, Difficulty XI is definitely not ready for primetime yet. Maybe it’s a 1.2 thing that accidentally partially snuck out early in 1.1.1?