308 License Plate Frame Melting?

I have a '85 308 QV for about a month now. The plastic license plate frame is melting at the bottom. It appears to be from the heat coming from the screen between the bumper and rear of the car. Is this normal? The plate frame was on my '82 GTSi before with no problems. The only difference is the '82 had no cats, and the '85 is US spec original muffler setup. Obviously a metal frame will work better, but I just want to make sure that it isn't excessive heat. Car runs fine....no "slow down" light or any issues.