Screen dims automatically with auto brightness turned off.
When in a vehicle my phone will dim noticeably making it hard to make out things on screen when in games or apps with dark backgrounds. Auto brightness is off.
iPhone 7 Plus, iOS 13
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You'll have to turn off Auto-Brightness if your iPhone keeps dimming and you want it to stop. Open Settings and tap Accessibility -> Display & Text Size. Then, turn off the switch next to Auto-Brightness.