Grasp of the Undying not working as described

See title. Instead of being available to proc every 4 seconds, grasp of the undying is working as a stacking effect. After 4 autos, grasp is charged, but rapidly decays. Is this a bug, or is the description wrong? Considering most tanky champs don't have much in the way of attack speed, I can't see how the current iteration on live is useful.