For the bug:

1. Go to video settings, enable ambient occlusion.
2. Hit apply and close settings. (You should be able to notice a difference in shadows on trees and other objects, it's pretty obvious when it is enabled)
3. Now, with ambient occlusion enabled, go to the video settings and change your gamma.
4. Now that you changed your gamma settings (increase or decrease), you will notice that ambient occlusion is no longer working.
5. Observe that ambient occlusion is now disabled, regardless of it displaying that it is enabled.
6. Try to disable and re-enable ambient occlusion and observe that it does not work.

To get Ambient Occlusion to work again:

1. Make sure ambient occlusion is disabled and broken (steps above)
2. Go to video settings, change Overall Quality to something other than what it was previously set to. (i change it to normal and then back to high, just change it a couple times)
3. Go back to ambient occlusion and enabling it should actually work now.

If my description or reproduction steps were unclear, just let me know and I will attach a video or something.

Graphics Card: GTX 760
CPU: AMD FX-8350 8-core
OS: Windows 10 Home
RAM: 16 GB