Another troubleshoot :
my EEPROM had problems, on statup some value changed randomly. Wow !!!
I've send a minimal program to arduino (only eeprom writing on a program, then another only reading) and check changing at startup -> Yes EEPROM was changing randomly at startup (plug/unplug USB cable) with random values in random EEPROM addresses.
Then I've putted my Nano off the board : everything was working fine.
I've find the guilty after removing MAX from the board : oversized Capacitor (220µF instead of 10µF) caused a voltage drop when shut down the arduino, which causes a writing problem in the EEPROM (Can Atmega328 corrupt EEPROM without a write - Microcontrollers - Arduino Forum).
5V was coming from 5V Nano pin.
Don't know if it helps someone but I've spent some time on it so it's cool to share it and help a hobbyist like me.
Edit : In fact I think there is a better option : put a diode in 5V output Arduino to avoid Capacitor to became a power source when shut down