Basically when you transmog your Main Hand with the Warlock legion artifact main hand, Spine of Thal’kiel, the off hand, Skull of the Man’ari also gets applied to the Main hand. The main hand ends up with two transmogs on it.
You can see in Armory pages that the MH has two transmogs applied in the tooltips on those effected by it.
A thread was started about this when the bug started with S4 patch, but it’s now locked.