Well, if you want to try something weird for the heck of it...

The sensors might be getting reflection from the back of the license plate. Put something that absorbs ultrasonic waves, and it might be just enough to prevent false alarms. I'd try sticking some of that self-adhesive insulating foam strips used for general heat sealing around openings in houses. Or cut from a sheet of foam and glue it onto the back of the plate near the sensors.

If it is reflecting from the edge, that a problem, as any foam or anything would be unsightly.

It's a bit of a crazy idea, but might be interesting to try.

You never know...