I recently installed the macOS high sierra developer beta 2 on my 2011 macbook pro 15 inch. The beta 1 was fine, I did not encounter any hardware issue such as this, mostly the fan spinned up quite a bit and the machine ran really hot. But since I updated to beta 2 yesterday, it's now running at the lowest possible brightness all the time. I turned off auto brightness as well as manually adjusting with the f1 f2 keys, or the controls in system preferences, nothing changes. The keyboard on the other hand is on full brightness the whole time too. It doesn't seem anybody is experiencing this, which is unfortunate. I just reinstalled the beta 2 again and it's still the same. Maybe I'll downgrade to Sierra, just to confirm it's the beta that's the problem.