Well, i have never seen +Mag on HA or LA. It should be AA
I looked on my Psy, and he gets the +40 Mag.
But like u wrote, u are using both, Primal and Demonic.
Thats means u are getting the full Setbonus of Magic Armor. It is just not displayed. That means that u will get those +40 even with 1 Demonic Piece and Primal.
And thats why u cant see a Decrease on Magic if ure only disarm ur demonic or primal. Disarm both and see what happens