Complete fresh install, no plugins. Add full fuel to Cessna Skyhawk either by typing or using sliders, metric or imperial, engines switched off.
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On the runway, prep electrical systems, fuel resets to default value.
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Again add full fuel via config screen, fuel shows up on fuel gauges,
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start engine, fuel defaults again 
So this is the Steam XPlane 11.10 on Mac, completely fresh install, no plugins.
All help appreciated!
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Update:
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.