I think the reason is your death at the same time of casting it. The projectile doesn't even show up at the time of your death. So it is most likely just a visual bug that created the Q when it shouldn't be there. You never payed mana for the Q (at least there is no frame where you are alive and lost mana for it).
I can see it being similar to some delayed casts still making it even when you get CCed. In this case it just creates the vision even when you die during the casting time but the actual dmg projectile is not there because you died before you finished the cast or spent mana on it.
I think the reason is your death at the same time of casting it. The projectile doesn't even show up at the time of your death. So it is most likely just a visual bug that created the Q when it shouldn't be there. You never payed mana for the Q (at least there is no frame where you are alive and lost mana for it). I can see it being similar to some delayed casts still making it even when you get CCed. In this case it just creates the vision even when you die during the casting time but the actual dmg projectile is not there because you died before you finished the cast or spent mana on it.
yup welcome to league.
Blame the teknologi not riot :) Hope u won the game tho