I wonder if somehow dhclient is being added to omitpids.
If you edit /etc/init.d/sendsigs, and uncomment the '#report_unkillable' on line 132, then reproduce agian. Then look in /var/crash for apport reports mentioning dhclient. If there are some, then its being added to the list of pids the system is not allowed to kill, but whatever has added it has not managed it properly.
I wonder if somehow dhclient is being added to omitpids.
If you edit /etc/init. d/sendsigs, and uncomment the '#report_ unkillable' on line 132, then reproduce agian. Then look in /var/crash for apport reports mentioning dhclient. If there are some, then its being added to the list of pids the system is not allowed to kill, but whatever has added it has not managed it properly.