This is not just for X-Mog only, wearing the helm itself have also stopped showing the wings visuals.
It used to block those visuals 9/10 times when using Pepe but reloging would bring those wings back with Pepe also nesting on top of your head. Since this week’s maintenance tho, wings visuals are simply gone. I still had that helm in the bank and despite wearing the actual helm, wings are no more 
Edit:- Since it won’t let me post a reply without another post, I have to edit this one.
My Warlock died in Devles and suddenly Wings started showing up again! I will update/edit here, in case if they go missing again. Looks like the “trick” is to kill yourself for these Wings to make a triumph return. 