Why does my Mac keep dimming even with auto-brightness off?

Your Mac isn't broken. Three macOS features dim the screen on their own even with auto-brightness off. True Tone shifts color and perceived brightness to match the room, so turn it off in Settings > Displays. Low Power Mode drops the backlight to save battery; check Settings > Battery, since it can switch on by itself. And on a 14"/16" Pro, sustained bright or HDR content in a warm room makes the display roll brightness back to stay cool. A warning triangle shows in the menu bar. Wipe the ambient-light sensor too. If you also want past the everyday cap, MacBrightness's free Splitscreen Mode puts the brighter screen in front of you first.