Weird anti-aliasing bug ?

[HornetMaX]

I start GPB with the following graphics settings:

• 1920x1080, fullscreen, 32bits, 120Hz, no vsync
• AntiAliasing set at 8x
• Scrren ratio = resolution, Texture size = high, filtering = triliner, aniso = 16x
• Dynamic shadows yes, Reflections yes, Shaders yes
• Draw distance high, model detail high, 3d grass yes

In the main menu page, I have the Murasama (default paint) showing, like this:


Switching the AA to 16x I get this (look at the front brake disk, it becomes somehow transparent, plus the dark parts become "weird"):


Now, if I switch back to 8x AA, the problem does not go away.
But if I then quit GPB and restart it (the AA is again set at 8x) I'm back to the initial situation (AA at 8x and no problem).

If it matters, I have an NVidia 970.