Buzz
According to some humorous comments on the 'bug saint' Vandiril's channel, it's highly likely that Kalista only 'reincarnates' into another timeline rather than being completely defeated. Thus, this champion can use such skills because it's absurd to use skills after being defeated.
However, a more convincing explanation has been put forward, suggesting that Mordekaiser's ultimate somehow slows down the server's processing speed. So when Kalista presses R before Mordekaiser is defeated, it's only after that moment that the ultimate is released. This isn't too far-fetched considering there are numerous bugs that cause the game to crash, even deleting the entire match history.
Hopefully, Riot Games will soon have a plan to address this issue as soon as possible. It would be terrible if such a serious bug were to persist in the game and affect major League of Legends tournaments.
