My iPhone screen stays unusually dim even though the brightness is turned all the way up and auto-brightness is disabled. I’ve tried adjusting settings, but nothing seems to work. I need help understanding why this is happening and how to fix it.
iPhones just love playing this fun lil game where they pretend their brightness is maxed out, but instead, it looks like they’re on ‘energy-saving mode for bats.’ 
You’re saying no auto-brightness, full slider up—so we’re ruling out user error (not that I’d accuse you, don’t worry). Here’s the breakdown of possibilities:
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True Tone Drama: If True Tone is on, it tries to adjust colors and brightness dynamically. Sometimes, it gets too clever for its own good. Check in Settings > Display & Brightness and turn it off to see if it makes a difference.
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Low Power Mode Nonsense: Low Power Mode doesn’t officially dim your screen, but weird things can happen when it’s on. Make sure that’s off.
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Overheat Reaction: If your phone’s been chilling in the sun or running a bunch of demanding apps, the iPhone basically goes
and dims the display to stop itself from becoming a crispy potato chip. Try letting it cool down. -
Accessibility Settings Party: There’s this sneaky ‘Reduce White Point’ setting hiding under Settings > Accessibility > Display & Text Size—if that’s on, it artificially dims the brightness. Sneaky little feature.
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Screen Tint/Grime: Sounds ridiculous, but sometimes, people realize their screen protector or a layer of dirt/grime is the culprit. Remove anything extra on your display and give it a clean (gently, don’t go crazy).
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Hardware Nightmare: If none of the above fix it, you may have a busted backlight or screen issue. Apple might need to step in here ($$$ vibes, sigh).
Try those, and lemme know if your screen learns to behave! Or just return it to Tim Cook himself and demand justice. 
Hmm, so your iPhone is playing this overly dramatic “dim screen” role and @shizuka already came through with some solid ideas, but lemme throw in a few other angles.
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Night Shift Betrayal: You checked True Tone, but did you peek at Night Shift? Settings > Display & Brightness > Night Shift. When that’s on, it sometimes softens the brightness weirdly, even during the day. Maybe it thinks it’s candlelight dinner time?
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Software Bugs Gonna Bug: iOS updates can be a blessing or a curse. If you’re not running the latest version, bugs might be messing with your brightness. Update it—or if you’re on the latest, well, sometimes the best tech companies accidentally release a lemon. Try a ‘Reset All Settings’ (Settings > General > Transfer or Reset iPhone). Painful to reconfigure, I know, but might do the trick.
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Battery Aging Drama: If your battery’s aging or damaged, it can freak out the display’s performance. Check battery health under Settings > Battery > Battery Health & Charging. If it’s below 80%, dim screens could be its passive-aggressive way of asking for a replacement.
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Third-Party App Interference: Certain apps (hello games or video editors) can override brightness settings. Are there any sketchy ones you’ve downloaded recently? Uninstalling one by one to test might help, but yeah, it’s tedious.
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Face ID Sensor Blockage: Random thought, but if you’ve got the sensor slightly obscured (cracked screen protector, dirt, whatever), the phone freaks out and dims the display because it thinks it’s protecting you from prying eyes. Yeah, “privacy” can backfire.
And about @shizuka’s ‘busted backlight’ theory? Possible, but let’s not call the hardware broken yet. Apple support = $$$, so wildcard option: Boot to recovery mode and restore the software. Painful, yes, but it rules out corrupted system issues before you start chucking money at it. If nothing works after all this, well—join us in the Apple tax club. 