@Black-Square Ah no... I spoke too soon...
I have just completed a 1 hour flight and turned off the Av Master before engine shut down. Everything was fine until I shut down the engine, then the old problem returned. So, it's not specific to the Avionics Master after all.
When I shut down the engine the only things left turned on in the aircraft were the battery and generator.
Referencing your last note, it seems that whatever is the last or nearly last action performed at the end of a flight, is what triggers the reset, regardless of the order. Either way, it's really putting a road block up regarding my enjoyment of this aircraft.
I hope you can sort this out.
Best wishes,
Luke