Question about Goobay Digital energy cost meter Pro

When you unplug the device, the screen goes blank and when you press a button, the saved value appears. The instructions say that after 20 minutes the saved value disappears and goes to 0, but on my device the value always stays there even after days and you have to press reset and re-enter the kwh price. Do all your values go to 0.0 when you have unplugged and waited a while?