Brightness control

understand that although you have Auto-Brightness disabled on your iPhon, you're still seeing the screen adjust without your doing so. I can see why you'd reach out about this behavior; I'd like to help. Since you've already verified that Auto-Brightness is disabled in Settings > General > Accessibility > Display Accommodations, I'd recommend reviewing the information below regarding your device and it's operating temperature as this can affect the brightness on your screen:

"If your device gets too warm

If the interior temperature of the device exceeds the normal operating range, the device will protect its internal components by attempting to regulate its temperature. If this happens, you might notice these changes:

• • Charging, including wireless charging, slows or stops.
  • The display dims or goes black.
  • Cellular radios enter a low-power state. The signal might weaken during this time.
  • The camera flash is temporarily disabled.
  • Performance slows with graphics-intensive or augmented-reality apps or features.

Additionally, if you're navigating, the device might show this alert and turn off the display: "Temperature: iPhone needs to cool down." Navigation still provides audible turn-by-turn directions. When approaching a turn, the display will illuminate to guide you through the turn."

Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures

If you feel that the brightness adjusting on your display occurs outside of these conditions, reach out to Apple for further assistance ☞ Contact Apple Suppor

Hey there,

this is Unexpected behavior try restart your iPhone ☞ Restart your iPhone - Apple Support

also make sure your iPhone is up to date ☞ Update your iPhone, iPad, or iPod touch - Apple Support