You need to set some of the overrides in the FPostProcessSettings struct too. It works similarly to when editing the values in engine with the checkboxes.
This is actually mentioned in the header file for CameraComponent:
/** Post process settings to use for this camera. Don't forget to check the properties you want to override */
UPROPERTY(Interp, BlueprintReadWrite, Category = PostProcess)
struct FPostProcessSettings PostProcessSettings;