Fiora ult bug

I thought that the intention of Fiora ult was to become invulnerable after casting it, but compare it to Zed ult, Fizz Playful/Trickster, and Yi's alpha strike, it is not. All of theirs is immediate and never failing! Sometimes when using it it can be stopped by CC because there's a short window of time after casting it that it can be stopped, but it can be recasted, however this is useless because by then it's likely you're being CC-locked. When you press the ult and you can hear the first sword 'sheenk' sound, it should not stop at this point. It's very frustrating and cost me a game.