[Richard]

It is either what Gordon says, in that the alternator on the genset is producing enough voltage that the viktagge regulator on the engine alternator thinks it doesn't need to activate. In which case current will be flowing slightly backwards through the ammeter, which will trip the warning light.

Or, the inverter charger is kicking in when you crank the generator, again providing higher voltage than the regulator on the alternator wants. This will cause the ammeter to read backwards. Any chance the batteries are merged?