After several weeks, it finally works : it seems that an optimization through GeForce Experience solve the problem !
Thanks for your answers.
Apart from GeForce Experience being a ridiculously intrusive hard-disk trasher, I never understand how it's supposed to work to begin with. Like won't its GTA V profile interfere with your own game settings? (Like it may decide to do MSAAx4 itself as well, on top of what you already have defined in your video settings, etc.)