[16.04] Cannot download packages whilst offline - when using ifupdown

I am using 16.04 with the main ethernet interface being managed by ifupdown, and others by NetworkManager. Apparently PK's pk_network_get_network_state() does not properly recognize this and thinks it is offline:

# pkcon update
Getting updates [=========================]
[...]
Fatal error: Cannot download packages whilst offline

A workaround is to change /etc/NetworkManager/NetworkManager.conf [ifupdown] managed= from "false" to "true", then "eth0" appears in "nmcli d" and PackageKit is happy.

In 17.04 this is no problem any more, PackageKit works out of the box with the same ifupdown configuration and managed=false. Interestingly, "nmcli d" shows

eth0 ethernet unmanaged --

there, while eth0 is entirely absent in 16.04. There doesn't seem to be an nmcli command to show what PackageKit looks at; the closest is "nmcli g" but in all three cases above that says

STATE CONNECTIVITY WIFI-HW WIFI WWAN-HW WWAN
connected full enabled enabled enabled enabled