I was playing as Cassiopeia in a custom game. I bought Archangel's Staff. When I scrolled over it, it wasn't the stacks that were bugged, it was the ability power bonus. I had 648 stacks and it said "Bonus: +0 Ability Power". However, I was still getting the bonus ability power I was supposed to get.
1). Play as a champion that utilizes Archangel's Staff (I chose Cassiopeia)
2). Get some stacks on it
3). Scroll on it and notice that the stacks are showing but the bonus ability power is not.