Although flight mode is activated, altitude acclimatization is displayed after flight.
I think flight mode only turns off things like wi-fi and Bluetooth. I think it's valid that you get acclimatization as the cabin is usually pressurised to the equivalent of about 2000m.
I think the altitude acclimatization should only be for sporting activities
But your body acclimatizes outside of activities. Like when you're sleeping. Take a read of this:
support.garmin.com/.../
