Gameplay Multiple 25 stacks dark seal

I was playing and purchased with after reaching 10 stacks contines to stack to 25. Unsure if it applies bonus ap with extra stacks or if it's just a graphical error.

1. purchase both and
2. proceed to stack as normal with champion kills 3.after 10 stacks contines to read past 10 to 25

Have seen it stack in two games both times, purchasing seemed to reset to 10 stacks