It will throw a DTC code because the evaporative system is not air tight.
Further, there is nothing to prevent gas from evaporativing while the car is setting. The outside temperature will affect how much gas is lost via evaporation.
It is not hard to replace the donut. They cost <$20 from the Ford dealer. It takes less than an hour to drop the tank when it is empty.
This is a common problem on older Mustangs. Fix it right and be done with it.