It's normal to have the car move in drive on a flat surface.
When you have the brake applied while is drive the torque converter is allowed to slip and let it stay still without moving and when you take your foot off the brake it that slip turns into a forward movement because nothing is holding it back.
That's the non fancy way of putting it. All regular automatic cars will do this. Some have a hill assist package will be keep the brakes applied for a few extra seconds to hold the car still but anything with a torque converter will move under it's own power on a flat surface.
