Bug with GA and Kassadin's riftwalk's mana cost (Game Break Bug)

So I played a normal game with my friend and he was kass, it was his first time playing kass and at the end of the game he told me "Oh my god, GA is so op, it resets the mana cost of Kass' R" He thought it is how it supposed to be.

I tested it out in a custom game, the result is that: GA resets Kass' riftwalk's CD no matter how many stacks it had, the mana cost just immediately** goes back to 75 ( 0 stack)**