I am not sure if this is a bug or timing issue. In the video you can hear Sona's ult go off and you can see her cooldown timer start but she never actually ult's anybody (it showed up on my screen playing Sona) but did not actually DO anything (and you can see from the replay that it indeed did not do anything).
I guess this is just how it's programmed. Much like Lux's R used to be. You could cast your ult right before you died and it was put on cd and the sound effect could also be heard but the ult would not go off. They changed it but it was more like a quality-of-life change rather than a bugfix. Looks like Sona needs one too. (sry if i misused some words/phrases, i'm PL ^-^)
