I had also noticed 'something' felt a little 'not quite right' but hadn't looked into it.
I might be wrong, but I don't think the regions get brighter after they have played, but the waveform changing colour (?) getting brighter?) possibly just makes the region colour look brighter.
Either way, the overall behaviour feels a little 'back to front' to me. In general, I would expect the played section to get 'duller' in some form.
It might simply be a case of reversing whatever changes on the waveform (e.g. unplayed = white/bright, played = grey/dull)?