It looks like trying to transmog legion artifacts is bugged to varying degrees in TWW. In my case, trying to set the artifact appearance to the main-hand will completely hide your off-hand weapon, no matter what you set the off-hand to. As in…I just straight-up don’t have a weapon in my left hand anymore. I should also note that this effect persists even after reverting the changes I’ve made, or applying a different set. My character just doesn’t have a weapon anymore… It appears to affect the weapon slot itself because swapping weapons still only hides the off-hand.
Re-logging appears the only thing that fixes this issue. I haven’t been able to test whether other players see the weapon in my hands or not.
Additionally, the “Legion Artifact” drop-down only shows a small portion of all the appearances I’ve unlocked, but all appearances show up under their respective weapon type drop-down (eg. only some Doomhammer appearances show up on the Legion Artifact page for my shaman, but they ALL show up under the 1-handed maces page).
The Legion Artifact page occasionally disappears entirely as well if you select certain appearances.