I looted the Diabolic Robe item from the Tomb of Sargeras Raid on one of my alts, however the chest piece transmog was not unlocked when I looted it. I do not see it available on my transmog list for my warlock. I have acquired every other piece for the Diabolic Raiment set (I farmed the Raid on multiple different characters) but the chest piece transmog was not unlocked even though I looted it.