Hi @FPode.1 ,
I unfortunately don't have good news about your question... the PIN 15 is used for programming and erasing the internal flash memory, since the HV levels needed for this purpose are generated using this supply. From internal check, I can say you that it is mandatory to keep the pad level over 1.8V in any functional mode. Max voltage on this supply is 13.5V. When not used for programming purpose this pad must be kept to power supply (VDD).
If you could invert the polarity of the reserved pins, i.e. both PIN10 and PIN15 to VDD level, things may be more safe for a standard working mode of the device...
-Eleon