In an attempt to rule out water causing a short in the fuel gauging system, when your tank is half full, take your car to a self service car wash and completely wet the exterior, maybe underside too. Before drying, see if the problem exists. Modern cars reset the fuel related readouts upon refueling. Maybe this caused the system to come back to "normal". I don't know if the NX lets you over-ride the reset so that if it happens again, maybe the fuel gauge won't show full when you fill up. Many systems are designed by OEMs, so other manufacturers could be affected as well. I also would not rule out a software glitch. Hope this doesn't happen again.