I did see that thread, but no one else has flagged this behaviour with the network connection being lost. Stopping sddm results, not surprisingly, in the same behaviour.
I should have said that a "systemctl restart ifpugd.service" brings the interface back up after it has been lost, so is this an ifup/down bug in systemd, or just my install is screwed up. I'm never going to remember to do a "apt-get dist-upgrade -d" only in graphical mode before switching text mode, so I'll just live with the minor incovenience for now.
As it happens I have a bigger problem with suspend at the moment.