I'm having this issue with my Galaxy S6 since I updated to Marshmallow.
Under direct sunlight it will rapidly jump to a higher brightness setting independently of having auto brightness on or off. I also can't manually reach the maximum brightness that the smartphone shows me when it's under direct sunlight (even with the brightness set to max). I can only control 75% of the max brightness. There's a thread on galaxys6 reddit that talks about this problem and has a link to a video (I can't post links yet).
I'm pretty convinced this is not a faulty device as this didn't happen with Android 5.1.1 and I've bought this only 3 weeks ago. I've already installed and changed my firmware to the latest France region (XEF) version and it's still doing the same.
Does anyone have this same issue? Is there a way to fix it ?