Okey... It's actually hard to blame Erwin about the controller quality. His electrics provider genuinely believes that everything is fine with the current HW.
I received fast and polite response from him:" Your testings confirmed never a voltage above 6.3V - so all ok so far." and then he referenced to Absolute Maximum (stress) ratings being 6.5V.
This also confirms that he uses ~6.3V zener solution by purpose and it's not just single batch manufacturing mistake. I tried to politely refer to Functional ratings being max 5.5V and Microchip is not guaranteeing functionality beyond those.. then he responded nicely:"Thank you I can read Mr. Clever".
It's rather clear that there is now an electric system designer who clearly doesn't understand how to read datasheets and therefore I know now one company where you really cannot trust on Electronics quality. If the designer doesn't understand the difference between Absolute Maximum ratings, Operational ratings and associated warnings in the datasheet I really doubt the overall quality of the products... But since I haven't bought anything from him directly it's not my business. This discussion seems as dead end. Everybody can draw their own conclusions I tried my best.
Ilkka