Problem is somewhere in relation between Unity engine and Arch Linux. Try to google your problem and maybe you will find solution.
Edit: Maybe this is your problem:
Hello again and thanks for your replay.
The game is a unity game and doesn't need wine for it to run but what's weird is that the mouse sensitivity works fine on the menus after launching the game.
I think it's a linux issue or a compatibility between unity and libinput or something like that
I'll keep you updated
Thanks again.