I have my 2015 iMac Retina 5k set at maximum brightness, and every time I reboot my computer, the brightness goes to 50%. It does not save my settings. So I also have a 2012 iMac with the same High Sierra beta installed. The same thing happens on this iMac. I have two MacBook Pros retina laptops and they work fine with the the brightness settings. So this is only related to the iMac computers. Is anyone else seeing this problem?