This one won’t go away in my head so have to post this theory here to be shot down.
Aircraft encounters cb cells, becomes covered in super cooled water droplets, temperature changes or encounter with ice crystals turn this to ice over much of the fuselage…ice blocks static vents, pitots may be unaffected.
Crew aware of an airspeed problem but altitude and vertical speed appear stable.
No mayday sent, no cabin announcement.
