Why is this happening ?

This is a bug that occur a lot to me and it makes the game difficult. In this situation the Teamfight was Instantly won by all the positioning of the enemy. I was Taking 3 or 4 of them with my ult (maybe they would not have died but a chunk of hp was going off). But instead The ult went the direction oposite of my cursor denying a really high damage. Why is this happening ? This kind of bug make me wanna quit the game since its literraly gamethrowing.

You can see thro the video that my cursor was following Ezreal 1 sec before i ult. At the moment of the ult my cursor gone from ezreal to lee sin. During the animation it was still near lee sin (just on top of sion portrait.) And after the ult the cursor was still on the left side of the screen. The ult tho, went literally on the opposite side (Right side of the screen). It make games hard and even can make players call you name for those kind of things.