I'd guess the reason for KillMode=process was the following: On update, you want to restart NetworkManager, but not break the network connectivity if possible [and thus probably the update]. A SIGTERM sent to dhclient would cause that it deconfigure the interface, except it is stopped using "dhclient -x ifname" or kill -SIGKILL. I'd say, we can reassign this bug to NetworkManager to handle the stop and update cases properly.