iPad keeps dimming auto brightness is off.

Perhaps your iPad is running too warm - and its thermal management is protecting the iPad.

Quoting directly from Apple… If your iPhone, iPad, or iPod touch gets too hot or too cold - Apple Support

“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.”