iPhone Screen Keeps Dimming - all auto brightness features/True Tone/accessibility brightness turned off

Hi iphoneUser3000,

Welcome to Apple Support Communities!

We understand that your iPhone display dims unexpectedly although Auto-Brightness is disabled.

It sounds like your iPhone temperature is too high which would cause the screen to dim in order to protect it's internal components. You can learn more here: If your iPhone, iPad, or iPod touch gets too hot or too cold - Apple Support

If your device gets too warm

iOS and iPadOS devices have built-in protections to prevent overheating. If the interior temperature of your device exceeds the normal operating range, your device will protect its internal components by attempting to regulate its temperature. Here are some of the higher ambient-temperature conditions and activities to avoid, because they might cause your device to change performance and behavior:

• Avoid leaving your device in a car on a hot day.

• Avoid leaving your device in direct sunlight for an extended period of time.

• Avoid using certain features in hot conditions or direct sunlight for an extended period of time, such as GPS tracking or navigation in a car, playing a graphics-intensive game, or using augmented-reality apps.

If the interior temperature of your device exceeds the normal operating range, 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, your 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.

We hope this helps.

Take care!

iphoneUser3000,

That's correct, as noted in the previous article "Use graphics-intensive or processor-intensive apps, games, or features, including augmented-reality apps

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These conditions are normal, and your device will return to a regular temperature when the process is complete or when you finish your activity. If your device doesn't display a temperature warning, you can keep using your device."

Hope this helps!

Have a great day.