https://wiki.project1999.com/Players...r_Sets_by_Race
Ice Burrower Skin is "Velious Leather 1" which translates to "Cloth" on that section of the wiki. If you scroll down you'll see there are unique graphics for the 2 monk races and those only show on human and iksar, so illusions with armor that uses monk graphics will cause you to appear naked in those slots.