Double Archangel's Stacking Not Working

Hello,

Stacking two archangel's will result in mana going up to 20, back to 0, and then going up to 20. It appears that both archangel's are proc'ing but at different times, and not additively. This was on kha'zix.

Also, there is a noticeable delay until the mana bar is replenished after casting a spell affecting the viability of unique archangel's builds on champions like yasuo. Specifically, yasuo will have to attack twice (Steel tempest -> auto -> (mana bar not replenished yet) -> auto) before casting steel tempest again.

Thank you.