We have found this behaviour to be tied to power management settings in graphics drivers, which, when disabled, allow the screen to maintain a constant brightness level.
The subtle changes that occur when going from a dark background on the screen to a light coloured background have nothing to do with the 'Adaptive brightness' setting, but instead are a result of graphics driver settings. In that scenario, the brightness changes based on the room lighting. That setting works using the ambient display sensor on a laptop. This behaviour is a dynamic brightness change that is unrelated to the 'Adaptive brightness' setting in Windows.
These changes happen automatically after a couple of seconds, and they can also sometimes manifest as noticeable flickering. On a dark background, the screen will dim automatically, while on a bright background it will brighten. On some laptops, you may have noticed a change in brightness as you go from a screen with a dark background to a screen with a bright background.
