Brake Disc Pads problems of the 2006 Lexus IS350

2006 Lexus is 350 vibrated when applying brakes from highway speed, at 44k miles I took to the Lexus dealership twice and they found nothing wrong, I finally took it to my private mechanic and it was determined the rear rotors were warped, rotors and pads were replaced with new oem parts and the problem was abated. At 56k miles it began vibrating again from the rear rotors, currently (59k miles) it is getting worse. This is clearly a design defect, I have never had an issue with rear rotors warping like this in 32 years of driving.