Hey there, Gramadragon.
We understand that the display of your iPhone 12 Pro is adjusting the brightness by itself, even after you've taken some steps with your iPhone to stop it. To confirm, you ensured that both Auto-Brightness and True Tone are disabled, correct? If unsure, please follow the steps below to check:
- Settings > Display & Brightness > True Tone
- Settings > Accessibility > Display & Text Size > scroll down to the bottom to see Auto-Brightness.
We want to make sure we understand the steps you've already taken. Also, you'll want to ensure that your iPhone is being used within acceptable temperatures. If a device gets too warm, the display may dim the brightness as well. If you'd like, you can read more here: Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures
Take care!