Yeah, you're right
I followed many thread before posting here. Most of thread are dated from 2016 and some guys running my issue. They resolved it by repluging this f#!* livebox and maybe the others don't care about random disconnection.

I'm really motivate to resolve this. I attach my full dhclient log (clog /var/log/dhcpd.log | grep dhclient). As you can see, the longuest disconnection is 26 sept. I replugged my box in the evening.
dhclient.zip