Mac screen randomly dims watching fullscreen video content [SOLVED]

Documenting my journey here since i got ghosted by the senior engineers at apple on this issue and had to figure it out myself.

My issue was that when playing any video content fullscreen on quicktime player, firefox, or safari (and this would happen in safari or quicktime regardless of whether its fullscreen or not), my display would dim to a very low setting at seemingly arbitrary times, sometimes to almost no brightness at all, then slowly ramp back up to full brightness.

It would always happen at the same times during a given video--i was using the release trailer for Hades 2 as a testbed for this: https://youtu.be/l-iHDj3EwdI

Interestingly, google chrome didnt have this issue.

Going through the debug process with apple, we found a few things:

1. it shows up in a screen recording--meaning its happening in software
2. it behaves differently in different apps (firefox only in fullscreen, safari and quicktime always, chrome never)
3. it doesnt go away when starting in safe mode (which rules out any startup applications)
4. It DOES go away when you make a new user on the mac and play the video there--meaning its probably a user preferences thing

After going through some utilities for exporting and comparing user settings in mac (if you're interested, i was using rsync to copy /Library/Preferences and ~/Library/Preferences into a folder, then using the "plutil" command to convert the plist files to XML so i could diff and commit them to a local git repository) I found the issue:

The issue was the "Dim flashing lights" setting under Accessibility > Display. It was on for some reason (could have been me), turning it off fixed the problem.

Posting this in hopes that this helps someone else, since Apple was unable to diagnose this issue.