[Gameplay] Dark seal stacks do not apply AP

I just got finished with a game and it seems the Dark Seal is not functioning properly. Both myself and the opposing mid laner purchased the item. Stacks were give out properly for kills and assists, but there was no AP gain associated with the stacks (Tooltip read 15 (+0)). Once I purchased a Mejais Soulstealer the stacks transferred and began being applied properly.

1. Buy Dark Seal
2. Get kill or assist to gain a stack
3. AP does not increase

I've only played one game with the item, but it did happen to both champions using the item.