Complete fresh install, no plugins. Add full fuel to Cessna Skyhawk either by typing or using sliders, metric or imperial, engines switched off.
On the runway, prep electrical systems, fuel resets to default value.
Again add full fuel via config screen, fuel shows up on fuel gauges,
start engine, fuel defaults again
So this is the Steam XPlane 11.10 on Mac, completely fresh install, no plugins.
All help appreciated!
Started plane using manual procedure i.e. not using the menu option to prep the electrics and start the engine and it kept the fuel value correct. So it looks like a problem with the menu option.