[21/9 screen - Mac & Windows] Minions and Champions lagging in the corners

Hello. Since few weeks i've a got a nice 3440x1400 screen and it's fantastic, except on one thing :

The minions and and the champions are lagging when they are in a corner and it's a bit annoying in team fight as you can imagine. I will try to record nexts game to show you but it's a bit random so....