Hey DavidS.62,
Thanks for using Apple Support Communities. We understand you're having an issue with auto-brightness not working as expected. We'd be happy to help out.
To get started, you'll find everything you need to know about adjusting this setting here: Adjust the brightness and color temperature on your iPhone, iPad, or iPod touch
If you have a case on your iPhone 13 Pro, please remove it and test further. Also, ensure your iPhone 13 Pro is fully updated to the latest software. If so and the issue is persisting without any accessories involved, there might be an issue with the ambient light sensor on your iPhone, so it'd be best to schedule a visit in-store at an Apple Retail Store or Apple Authorized Service Provider for further evaluation.
For help locating and scheduling a visit nearby, check out: Get Support
We hope this information helps out.
Take care!