I'm no programming Wiz but Looking at it the issue seems to lie with it not updating quickly enough
To explain in programming theres a section which always loops, this happens once everything has loaded in, every loop it checks or changes various commands and classes, Say a champions movement is on a class the main loop will notice the changes and alter in game accordingly
Now what it seems to me is that the blastcone isn't updating as quickly as say, hitbox detection or character movement, this is apparent too with its indicator of where you are going to land, League isn't locked to a grid but the blast cone gives the impression that it is, odds are this is to lower the load on computers but it can causes issues LIKE it being delayed in registering that somebody has hit it.
Again I'm not gonna proclaim myself a programmer but from what I know this sounds like it might be it?