That seems to be a bit of a bug with regards to the objects/textures not rendering quick enough, I never had it when the game first released but one of the patches seemed to introduce the issue, seems to be random for me too, never had invisible enemies though.
Not sure on your rig but dx 12 provides a massive boost too, I run everything max @ 2560x1080 except the shadows, reflections and no AA (I find the game doesn't need any) and the sliders for distance and detail are about 70/80% and the game doesn't drop below 50 fps 95% of the time.