I'm having the same issue (brightness slider in right hand side disabled, the setting "automatically adjust brightness" just vanishes from the settings>display options, as well as my power button no longer putting the surface to sleep.
However I've realised that when I plug another monitor in so that my display is extended, the settings come back and I'm able to adjust the brightness of the surface pro screen. As soon as I unplug the additional display, all the surface brightness options
are disabled / removed.
The power button still however doesn't put the surface to sleep.
Is anyone else experiencing this behaviour?