Are you sure they were powder coated and not sprayed with a ceramic coating and then heat cured. I do a lot of powder coating. I did some headers and exhaust manifolds about 15 years ago with some powder that was supposed to be specifically for headers and withstand 1000 degrees (or something like that). I wound up refunding the money to each customer because the powder failed. A few years ago I found some ceramic paint that works really well. I sand blast the part in my blasting cabinet and cure it for a while to give it an initial cure. Then when the part is installed, the manufacturer's recommendation is to run at operating temperature for 20 minutes. If you did not have access to a powder coating oven for an initial cure, the guy I talked to from the paint company said you could do the entire cure on the car. It seems to work really well. The paint isn't cheap, but it works pretty well. I have not had a customer complain yet. I wonder what @CudaChick1968 has to say. I bet she doesn't powder coat headers or exhaust manifolds.
