Do you know if your dipstick tube is fully secure? It might have slid out perhaps, reading low when it's full?

On your oil change to 1l low for testing do you recall how much oil you put in and what the capacity normally is?

Just wonder if the dipstick could be wrong, though I trust that more than the sensor, you never know!

Also try checking the oil with the car facing down hill, on my 35d, it reads almost 1/3 lower at that angle, which might help you see if the sensor reads lower.