Is it a bug on Fiora's ult?

Yesterday when I was playing Fiora, I used my ult on Sion and killed him. The heal didnt work and the ult remained on Sion even after he respawned. I am wondering if it is a visual bug or not. If not, it wouldn't be fair for the game cause every time you meet Sion you can kill him easily with this bug. And I am not sure if this bug works on other champs like Karthus or Anivia. I've actually seen other players posting this bug, but it is still not fixed in this patch( At least Riot didn't write it out). Hope this bug can be fixed. (P.S: It is unfortunate that I didn't take an image when I was playing)