Chosen Solution
The brightness slider will work for the first 8th or so of the bar, and then the brightness will stop increasing and stay there. Auto-brightness is off, I’m fairly certain this iPhone has had a screen replaced recently as Touch ID is non-functional. Could it have anything to do with that?
Either it’s from a low quality replacement screen or the backlight circuit is damaged on the phone. I would try another screen first as it’s cheaper than the board-level repair option. iPhone 6 Screen Replacement iPhone 6 Screen
