Screen dimming while on max brightness on brand new iPad Pro (M4) with auto brightness/true tone switched off

I have a brand new 11-inch iPad Pro (M4) with nano texture glass. I use this iPad as a flight deck instrument, i.e. it is running the Foreflight app, set up with the flight plan, connected to the AHRS/GNSS inputs, and just left to display flight progress without any touch interaction often for 30-60 minutes at a time. When flying in daytime, I need the brightness at max for the display to be clearly visible. The way it is mounted on the flightdeck ensures it never is in direct sunlight (so solar irradiance/heat is not an issue), and the environment is always around 20-22C, never hot in cruise. The iPad also isn't hot (not even warm) to the touch when dimming. It is plugged into a power source (not running on battery). This also happens when just letting it sit at max brightness on my desk for testing this issue.

After exactly 5 minutes of letting it sit at max brightness and without touching, the screen dims by about 20%, enough that the screen no longer is as bright and as easily legible in the sunlit flightdeck.

I have switched OFF the following, but makes no difference:

Accessibility -> Display & Text Size -> Auto-Brightness

Accessibility -> Display & Text Size -> True Tone

Battery -> Auto-Brightness (seems to mirror the Accessibility setting anyway)

As soon as I touch the screen anywhere the brightness returns to max for another 5 minutes.

Any thoughts on how to prevent this from happening? What setting am I missing?