I've been experiencing this issue for a while now, and it seems completely local to my computer. There are three main champions that I'm experiencing issues with how their abilities are displayed on my screen.
Aurelion Sol: His stars are completely out of sync with the game. They'll rotate fine, but their hit box is completely out of sync. It seems to happen only after expanding them, which comes with its own graphical glitch where they jump about the screen when activated.
Ekko: His Q becomes invisible after it hits the point where it slows down, so I can't actually see whether I'm going to be hit by the Q or not.
Xayah: Her Q travels really slowly, completely out of sync with it's hit box. The feathers appear on the ground a good second before the ability even reaches the end of it's travel.
From the three skills I'm experiencing trouble with, it seems to be an issue with abilities that have varying movement speeds. Aurelion's stars speed up when W is activated, Ekko's Q slows down at the end of the ability and Xayah's feathers have a brief slow period before they accelerate.
Due to it being a local problem, I've tried repairing the client and it hasn't helped. Is there anything else I can try in order to fix this issue? It's made one of my favourite characters almost completely unplayable (Aurelion) and makes Xayah much more difficult because I haven't been able to learn how her Q is actually supposed to travel.