What I would do is take two vehicles to a gas station and take video of yourself filling up both vehicles and see the behavior of the pump and the two vehicles. If you can document this, it will help your case because it is normally a behavior of the fuel pump to shut off before it overfills the tank.
Once you have documented the problem, take it to the dealer and show them, and if they refuse to help, send the video to Hyundai and cc the dealer owner/manager.
If there really is a problem, you need to be aggressive to get it addressed.
Remember, the squeaky wheel gets the grease!