Tire Pressure Monitoring System problems of the 2010 Toyota Prius

I own a 2010 Toyota Prius. When I went check tire pressure the left front cover on the valve stem broke off in the threads of the valve stem just at the bottom of the cap. The tire still held pressure but the inner valve was exposed down to the point where the seal was. When I brought the car to tires plus the mechanic said this was common. He showed me a trophy jar with three other tire pressure monitoring valve stems identical to mine. I brought the car to Toyota and they replaced the tpm module in the tire. I work in aviation and have a background in maintenance and inspection. The four tire pressure monitor units I examined we cracked in the exact same place, right at the base of the dust cover. I suspect corrosion stress cracking. My car has 37,000 miles on it, no physical damage to the car or the valve stem was apparent.