A few days after I had upgraded to Sierra I noticed my rMBP's screen looks a little washed out when dimmed. I've always kept it on lower brightness when possible, as my eyes can get a little sensitive, so I'm pretty sure this is a new thing. When the display is dimmed, the OS seems to be trying to counter that by lowering the contrast, so the darkest parts don't get too dark. After you quickly turn the brighness all the way up, the contrast gradually returns to normal, over five or six seconds. Also, this behavior goes away with automatic brightness disabled. So, is this a new thing? If not, how long has it been there?