Hi cristiano214,
We appreciate you reaching out to us. We understand that you are experiencing issues with your device's Auto-Brightness feature.
We’d like to gather additional details that may help us be in a better position to assist you.
Do you have a case or screen protector that may interfere with your device's sensors?
Does it help if you enable the Auto-Brightness feature, then disable it again?
In the meantime, we recommend that you check for a software update.
While it isn't inherently assured that an update will resolve the issue, updating to the latest software version for your device can help isolate potential software issues.
Kindly secure a backup and update.
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
Take care!