That's interesting Pete. I'd assumed that it was the ethanol in the fuel making parts of the sensor mechanism expand and jam. That's assuming that the sensor is a float in a tube. These parts likely to be made of plastic and like plastic fuel tanks susceptible to absorb whatever it is in the ethanol that makes plastic fuel tanks expand and rot (as a previous MV Agusta owner I'm familiar with living in fear of this issue... ). Maybe, ironically, it's an additive in the Super fuels some of us use that's the culprit?
Millions of cars all have fuel level sensors in their tanks. They are mounted pointing vertically downwards, and never go wrong. Multistradas have the sensors installed upside down at an oblique angle. Is it any wonder that they continually stick?
