I just got the achievement Challenge Cave Delver III, for clearing 30 delves while playing on Seasoned difficulty or higher, while playing on Adventurer.

I haven't done much with difficulty settings, only switched to Seasoned shortly after it came out to complete the Golden Pursuit and get the Seasoned Daedra Slayer dye, otherwise I have just stayed on Adventurer, so I would be very surprised if I had completed more than a couple of delves on Seasoned at all.

I have only been on Adventurer with this character that I have been playing lately (levelling a dragonknight, if that is relevant at all). None of the other achievements linked to difficulty have progressed, like Seasoned Nature Slayer etc, so it seems to be a bug with this one in particular.

Seasoned Daedra Slayer is also bugged, incidentally, as I got the achievement and the dye while having bestial daedra at 47/200 and humanoid daedra at 33/200. The atronach ones were completed.