This makes sense to me. 18 minutes is a long time. Someone was not paying attention to what the aircraft was doing. Airbus has a record of control issues where the computer wants to do its own thing and of pilots not knowing how to overcome the computer's control of the aircraft.
This sounds like the flight was at the "set it and forget it" phase of the flight. Routine trip, daylight,waypoints already in the system, relax.
Someone was not minding the altimeter, or the altimeter was wrong.
If the crew were overcome by hypoxia, I would think the plane would just continue at 38,000 ft., as there is no other input to tell it otherwise.
Somehow this plane had a downward trajectory, and no one realized it until too late.
I wonder if ground control noticed and tried to hail the aircraft.
The Airbus is going to be called HAL9000 soon.
ATC called an “Emergency” for the flight when no response was received from crew and altitude was declining from filed flight plan.
Affirmative Dave, I read you !