Had this issue in iOS 8 and 9 a lot but it didn't reappear in iOS 10 for me - iPhone 5s. I always noticed that this bug happens when I was outside in full sunlight, maybe an issue with the brightness sensor and/or a bug in the software itself?
Found the problem. It was related to Zoom being turned on in settings. Turned it off and it brightened up big time. Thing is, I've had zoom turned on since iOS 9. Strange that it suddenly dimmed the screen like this after a reboot of the phone.