Hey 22M_00,
We're happy to help you figure out what's going on. You mentioned checking your brightness settings. To clarify, when you go to Settings > Display & Brightness, do you have True Tone enabled? If so, it's designed to automatically adjust your iPhone display based on ambient lightning conditions.
If you go to Settings > Accessibility > Display & Text Size, this is where you'll see if you have Auto-Brightness enabled. If so, turn this off. However, keep in mind that turning this off will affect your battery life and long-term display performance.
For more info, check out: Adjust the screen brightness and color on iPhone - Apple Support
Lastly, if all else fails, and this behavior doesn't appear to be related to your settings, it'd be best to contact Apple Support for further support: Get Support
We hope this information helps out.
Take care!