Ahem my mistake, I was talking about the fuel range. (might be the heat here at the moment...

)
The fuel gauge should be like you describe in your post, drop bar by bar.
So probably the tank unit might be faulty. Is the Software ever been updated and what version you have in the dash?
If it is been updated always make sure the mechanic sets all the parameters to 1290 SDR as the software is the same for 1290 SDR, ADV1190 and ADV 1050.