Hi Vincent,
I did not agree that we can only refer test data to ensure quality / reliability of component because of failure was found once used, it is exactly your reliability problem.
As your factory can not through the root cause why failure found only part from "F4 marking", My FA Lab had done that:-
1) found abnormality of marking F4. It has lower MnO2 than another marking ("F5", "F3", "D6" and "E9"), at area near Anode polarity which also found burnt.
2) found failure unit burnt at area near Cathode polarity when simulated over voltage applied.
Details are as attached. Please provide RMA# for all parts high risk within 48 hrs including with extra cost of this analysis. Importantly, corrective actions on how to solve / prevent this problem are really required.