Everything about Transmog
This original article originally mentioned that this was for all artifact weapons and this was incorrect. We apologize for the mistake.
With the Midnight Pre-Patch, all transmogs have been reset as part of an update to the new transmog system. Players have noticed that Artifact weapons are no longer able to be selected in the weapon dropdown menu, but are able to be transmogged if they match a weapon type that the character can use, even if they were part of a previous custom set. The problem is for weapons that are not part of a weapon type that players can use such as Claws of Ursoc for Druids as they are fist weapons.
It's likely that this is a bug, and for the time being you will have to use a different transmog on your weapon.
