It seems to be bugged, I don't believe any mask style pages are listed on guild traders.
I also did not drop the shoulder style page after opening far more Urgarlag's Mystery Coffer's than I would usually open to drop the shoulder page.
Chances are if one is bugged, the other one is too, that's why I posted.
I've yet to see a mask drop, nor any of my guildies/friends I've asked to, and I'd say 30-40 different runs combined; nor any seen in zonechat or guild stores yet.
I made a combined post for mask and shoulders in bug section here here