Just some reference information discovered:

Motion Sensors and 6-Degrees Orientation

~$ monitor-sensor
    Waiting for iio-sensor-proxy to appear
+++ iio-sensor-proxy appeared
=== Has accelerometer (orientation: undefined, tilt: undefined)
=== No ambient light sensor
=== No proximity sensor
=== No compass
    Tilt changed: face-up
    Accelerometer orientation changed: normal
    Tilt changed: tilted-up
    Tilt changed: vertical
    Tilt changed: face-up
    Accelerometer orientation changed: right-up
    Accelerometer orientation changed: normal
    Accelerometer orientation changed: left-up
    Accelerometer orientation changed: bottom-up
    Tilt changed: tilted-down
    Accelerometer orientation changed: normal
    Tilt changed: tilted-up
    Tilt changed: face-up
    Tilt changed: tilted-up
    Tilt changed: vertical
    Tilt changed: tilted-down
    Tilt changed: face-down

The following are the absolute positions reported by monitor-sensor:

• tilted-up: ~45 degree landscape (screen facing downwards)
• vertical: landscape (screen facing forwards)
• tilted-down: ~45 degree landscape (screen facing upwards)
• face-up: screen facing down
• face-down: screen facing up

The following are the accelerometer orientations:

• right-up: portrait with power LED, HDMI and power ports bottom-side, and power, volume buttons top-side
• normal: landscape with keyboard connectors pointing downwards
• left-up: portrait with power LED, HDMI and power ports top-side, and power, volume buttons bottom-side
• bottom-up: upside down landscape, with keyboard connector pointing upwards

Note the “face-up” and “face-down” values – it is almost as if the sensor was installed upside down?