I am having this problem on my shadow priest, on whom I generally apply the Xal’atath appearance to whatever weapon she has. I access the Xal’atath appearance by selecting “Legion Artifacts” from the drop-down in the transmogrify window.
Upon logging in after reset, her staff had lost it’s mog and I am unable to reapply the Xal’atath appearance to it. I was able to apply the appearance of T’uure, the holy priest legion artifact, by accessing that appearance from the “Staves” section.
I switched to a mace and found that I could not mog that to Xal’atath either.
I then switched to a dagger and was able to apply the Xal’atath appearance. It did not matter whether I accessed the appearance from the “Daggers” section or the “Legion Artifacts” section – either worked.
My resto druid is also having this problem. After reset, the dagger he is wielding lost the appearance of G’Hanir and I am unable to re-apply it. I am able to apply the appearance of Fangs of Ashamane, the feral druid legion artifact, by choosing “Daggers” from the drop-down. However, I cannot apply the appearance to his off-hand since he cannot dual wield as a resto druid.
My demo warlock is not having this problem yet. His staff retains the appearance of the Skull of the Man’ari. However, if I equip a different staff, I cannot apply the appearance to it. So he only retains the appearance until he loses it or upgrades his weapon. The main difference I can think of is that Skull occupies the off-hand slot while Xal’atath and G’Hanir occupy the main hand slot.
My prot paladin cannot apply the Truthguard appearance to a two-handed weapon even though Truthguard is also listed as an off-hand item. Unlike the warlock, the paladin did not have that appearance already applied before the reset. I can’t imagine there are many prot paladins out there using the Truthguard mog on a two-handed weapon, but if there are, I wonder if they also escaped the appearance removal.
EDIT: Apparently a the hotfix went out and now the issue seems to be fixed for everyone except the warlock. He retains the Skull of the Man’ari appearance on his staff, but cannot transmog another staff with that appearance. The main difference I can see is that Skull is an off-hand item where the others are two-hand items.