Hi Gorj ,
I never knew it locked when the engine is running ,,,,you learn something every day,,,
If it locked when the car was parked (and not locked ) ,,how would you open it to re-fuel ?
When you lock the car ,,the fuel flap locks at the same time...giving you security.
I did not know that. I have never tried to open the fuel door when the vehicle is locked. So the engineers are smart - the door is only open when the vehicle is unlocked and the ignition is off. That makes sense. A 2006 Mercedes has better technology than my wife's 2017 vehicle loaded with all the latest technology, yet you have to pull a lever to open the fuel door!! But I do like the adaptive cruise, the push button start, the navigation, and the lane departure warning in her car.