Brake Disc Caliper problems of the 2017 Toyota Camry

Rear back brakes are not safe they lock up. Toyota 2017 Camry purchase 9/2016. 8/2018 , had 5 pt inspection brakes were good 8mm all 4 at 27000 miles. Then 10/2018, left rear wheel screeching noises & wheel became extremely hot to touch after driving15-20 minutes. Dealer said it "was environmental", found that rear brake was frozen in bracket and sitting sideways. How did that happen? they did say it was the way I drove and environmental. They fixed it and cost me $500 I asked is this dangerous if brake freezes. ? 1 year later 10/2019 and only 11000 miles on the new back brake pads and rotors, the same thing happens to the same wheel, left rear. . I called dealer and brought car in. What a dilemma. Again they said it is environmental?? dust??. They reported that the the new brakes pads are now 7mm after replacing them in 10/2018?? how so ? it appeared that again the pads got stuck in the caliper braket which they cleaned up and lubed. Same problem in 10/2018 I asked why is this happening , could the caliper and bracket be defective. They said it is the material corrodes faster. They told me I should have a brake inspection cleaning every year for $125 as preventative. Now how does a car after just three years have another brake problem, the rear left wheel. All other wheels/brakes are good? so it is environmental for one wheel?? headquarters of Toyotas knows about this problem and telling the dealers to tell the customer that it is environmental. Also dealer admitted that there have been other cars same problem and Toyota hdqt know about it but not doing a thing. It scares me to drive this car with a potential braking problem that may cause an accident if the brake freezes and I am not aware of it and the brake pad deteriorates on a long drive. Toyota should owe up to the poor quality of material they are using cause a dangerous situation to arise.