It seems that while doing something for extended periods, like playing a puzzle game with no time limit, I notice that the screen is almost cycling through different brightnesses.

Now this isn't "lowering the brightness of the screen due to inactivity", I'm fully aware of that. This is happening while the screen is actively being used - it will suddenly switch to a slightly darker brightness - then a minute later, will spring back to full luminesence.

The "darker screen" is not as dark as a "power-saving mode" dark, but about halfway there. It's hard to describe. But it has happened repeatedly.

Might this be a power-saving function of the new firmware? Or sign of a bad screen, maybe?