Cloth heirlooms and Wind of Wisdom buff not stacking

Cloth heirloom shoulder and chest not stacking XP boost with Winds of Wisdom buff. Was testing with my wife’s goblin Rathnic and my goblin Lahnna on Frostwolf. When I took my shoulder and chest heirloom off I gained the same xp as when she had her’s on. Had her delete and recreate her heirlooms. Still didn’t stack.