Brightness changing without slider moving

As described, if consistently using high screen brightness, you may be encountering the iPad’s thermal management doing its job - in trying to protect your iPad, in particular the battery, from irreversible thermal damage. High CPU utilisation and/or high screen brightness will cause your iPad to generate more heat than normal - and with high ambient temperatures, your iPad may begin overheat.

You may be able to improve cooling by moving to a room and away from direct sun. If your iPad is in a case, try removing the case, as this will improve direct cooling of the iPad chassis.

Quoting directly from Apple… 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.”