Hi Fourzarfgg,
Thanks for contacting Apple Support Communities. I see that your iPhone 11 keeps adjusting the display brightness on its own. You mentioned that the setting for automatic brightness is disabled.
Do you have the True Tone setting turned on? If True Tone is enabled, the display brightness will adapt to the environment. You can find the True Tone setting by tapping Settings> Display & Brightness> True Tone. It's also a good idea to verify that your iPhone is up to date as well. The current version of the iOS is 13.5.1. Update your iPhone, iPad, or iPod touch.
The following resources explain in greater detail what settings effect brightness and how to manage those settings.
Cheers!