Same here.
Guardian Druid, had my staff mogged into the Claws of Ursoc, but cant do it now. I CAN mog my staff into one of the artifact staves in the staff menu, but cant mog it into the fist weapons from the Legion Artifacts menu.
This is very distressing. 
My Paladin’s mogs are fine: his mace is mogged into the mage tower Oathseeker through the Sword menu, and his shield is mogged into Truthguard from the shield menu.
Adding my voice to this issue as well
I too am having this issue. I logged in to find that my staff which was transmogged to Xal’atath, had it’s appearance reverted and when attempting to reapply the appearance, nothing happens; the sound plays but the appearance isn’t applied.
joining the club here
my druid can no longer transmog his 2 handed weapon into the claws of ursoc
this isn’t effecting every legendary mog though. my dk transmog blades of fallen prince and that’s working still
Same here on my druid. Can’t stand playing ugly, or better words, with an unpleansent look.
Same with me. I can xmog my MH/OH to one of the Scythe of Eline appearance but only the one specific one
yup, i cannot transmog my legion appearances
Same here, can’t transmog my Primal Stalker daggers on my kitty 
Hello? Devs? Where are you with a response and eta on a fix?
Same issue on Ret Pally. Blizzard please fix!
I’ve got the same issue on my druid. Hopefully a fix is coming
Adding my voice in because not being able to use my t-mogs I worked hard for is definitely time-to-move-on-to-a-different-game territory
yea my xmog is busted as well.
Após a manutenção quando loguei no meu personagem vi que o transmog de arma havia sido retirado, tentei reaplicar o transmog do Alunet porem não entra, quero que o erro seja corrigido.
I logged in today, after maintenance, and my Legion Artifact transmog was removed from my appearance in all specs. It’s still an option on the transmog vendor, but clicking okay does nothing.
Just adding my experience to the thread. On my Druid I have a polearm that I normal have transmoged into one of the Claws of Ursoc. Today that appeared to be removed. When I when to change it I could not apply the Claws to my weapon.
I attempted to transmog a random staff and that worked. I also was able to transmog to one of the Legion Artifact Weapons themselves (Scythe of Elune) in the staff category, but the Claws of Ursoc under the Legion Artifact option does not work. I have not tried any other combinations, but it seems like the items in the Legion Artifact area no longer ignore the weapon restrictions (allows fist over a polearm in my case).
This person claims their transmog is now working; is anyone else having the problem still? I just tried with no luck.
I did see another post with a point; the trial of style was last week. Could this ending have caused the problem?
This is just garbage, I noticed that when you go into the appearance tab, under legion artefacts, the other spec specific appearances once again say: "Requires specialisation “arcane” (as a fire mage).
Sounds like their code for keeping the weapons not spec specific any longer got borked with the new patch.
I have the same issue. After logging in around 6pm after a good days work at home, the first thing I noticed was that my legion artifact appearance had changed back to my currently equipped weapon and shield. Because I didn’t look legit, I mounted up on the Yak to change it back to the holy paladin legion appearance with no luck. I logged out and back in and even tried heading to Oribos thinking my Yak was broken. Nope, no luck there either. I reported the bug. Hopefully it gets fixed sometime this expansion?
I’d like to add that I am still experiencing this bug as well, and it most certainly started today–yesterday I had my dagger + offhand xmogged as Scythe of Elune, but now whenever I visit the xmogrifier I can select Scythe of Elune as an appearance, see the price, but the button to apply the mog is unresponsive. Hoping for a fix soon!