Failure Rate calculation based on manufacturing defects (capacitor)
So if you’re positively sure the failure comes from the vendor your original assumption that the capacitor will eventually fail given an exponential distribution is false. This distribution applies to random events caused by the component’s exposure to time and stresses. What you seem to be experiencing is a quality issue normally expressed in ppm by the manufacturer. What is odd is that from your explanation this is being unexpectedly high. My suggestion is to get in touch with the manufacturer and request for an analysis of one of your defective capacitors. At the same time check the ppm of this component to realize the correct failure rate you should expect before going into field.