bisogna mettere PostFX su normale,poi tornare a quello precedente da voi selezionato,se era ultra mettete ultra..
qui sotto la guida Nvidia che riporta il Bug,quindi praticamente io ho giocato fino ad ora senza Ambient Occlusion

Ambient Occlusion
Ambient Occlusion (AO) adds contact shadows where two surfaces or objects meet, and where an object blocks light from reaching another nearby game element. In Grand Theft Auto V though, the setting's a tad bugged, and is awaiting a fix to restore full functionality. Enterprising gamers, however, have found a workaround: change the Ambient Occlusion setting, Apply, change PostFX to Normal, Apply, change PostFX back to Ultra (or its previous value).
Using this method, we can now demonstrate the benefits of Ambient Occlusion High and Off, though Normal still appears non-functional, having zero impact on image quality. Rest assured, we will return to Grand Theft Auto V's Ambient Occlusion setting for a proper evaluation once an official fix is issued.