A little bit of internet "research" aka googling led me to another setting, that Apple very successfully hid under Accessibility > Display and Text Size. At the bottom there is another auto brightness switch.
But, I just tried that, after a couple of minutes brightness from full went down a notch (still ok), and then another (not so ok). So that switch didn't help either.
Closed Aerofly, and while writing this, brightness returned to full. Battery is almost full. Somehow there seems to be an inbuilt protection of some kind....
Patrick