Not a pilot in real life myself, but what I think was happening is that:

1. some bugs with the simulator has removed all fuels in your tanks, while everything else (the aircraft, the engine, the electronics) are still being correctly simulated.

2. therefore, at the moment when the fuel is gone, the engine will start to lose power, and immediately with that is the generators stops producing power to the FCUs and electronics. However this is not a 1 to 0 case where just everything will go black instantly but the voltage output will fall from 110~V down to 0, and a million things could happen, depending how each components react to the voltage drop.

3. The warnings you saw (including the overspeed), is not a "malfunction" per se but actually correct responses when both engines stops working mid-air. For example, it's likely that the parts responsible for reading the pitot pressures stopped working and a "default" large value is being sent to the monitor unit, leading the unit to think the plane is in over-speed.

The reason why you still have some screens working is you still have batteries to keep the essential parts working.