Most likely you've run into the 'Engine started to maintain fuel system' setup.
The car expects you to put about 20 litres through the engine every six months (maybe every 3 months in the USA).
If you only run the car on battery, the system will force you to use petrol, until you've added another 20 litres to the fuel tank.
This is to maintain the engine, seals etc. in the fuel system, and stops the fuel in the tank from going 'stale' or gelling.
It's similar to the requirement that the air-conditioner be run at least once per month.
If your tank is already full when this happens, you just have to drive around in fuel mode for a while, until you have enough space to add more. (In my case, with the low mileage I do, that took about four weeks)
I've changed my weekly errands etc. so that I use a little bit of fuel every week, and haven't seen this mode for a couple of years now.
It's possible to clear this state by disconnecting the battery for a period of time, but generally I don't recommend that owners try to second-guess the Mitsubishi engineers.
