Other Fuel System Tank Assembly problems of the 2018 Chevrolet Malibu

When I'm sitting at a stop light awaiting my turn to proceed with my foot on the break the car is fine. After sitting at the light and it's time to drive, I remove my foot from the break and my car shuts completely off. When I apply a little gas the car turns back on and it malfunctions even more. My dash board goes haywire with different error messages. It says the fuel is low which it is not, I could have a full tank. The back door is unlocked, I've never seen that message before until the malfunction. My blinker tone even changes and my car is reluctant to accelerate. This has occurred twice now.

The car stalls while I'm driving on highways regular streets turning no check engine light car full tank proper oil levels etc.

I purchased my 2018 Malibu in August 2018. I was getting an average 28 mpg until my first oil change in December 2018. After that, I started getting 20-27 mpg city and 14-20 mpg highway. This car will use an entire tank of gas going approximately 210 miles on the interstate. I have taken it back to a certified gm service department, and they tell me that the car has no service codes. I have spoke to a gm manager, and she told me basically the same thing. I have been told that nothing will be done to correct the issue with my car. I'm not understanding why I got excellent gas mileage from the car during the first 6000 miles and now it fluctuates on city roads and is awful on the interstate. I have never drove a car that got better city miles than highway. The car is also very hesitant and shifts hard at times when accelerating. This is not an every day occurrence, but happens 6 or 7 times a week.