Sounds like a damaged sensor on the phone - when did you get it, is it still within the warranty period? also what happens if you take it off auto and go outside, does the problem continue?
It's not a damaged sensor. It has something to do with marshmallow and the auto brightness. It started happening the second I installed marshmallow. Prior to that for 8 months auto brightness worked perfectly.
You won't find an answer to your problem. My thread about this 2 months ago magically got deleted.
You'll just have to deal until Samsung fixes their **** which probably will be never.