Some devices (namely PPPoE (pppX), ADSL (nasX, pppX), and
mobile broadband (pppX, bnepX)) create a kernel netdevice for IP
communication (called the "IP interface" in NM) as part of the
connection process and thus the IP interface lifetime does not
correspond to the NMDevice lifetime.  For these devices we must
ignore removal events for the IP interface name otherwise the
NMDevice would be removed.

Caused by 8cce42f2.

For example, this bug caused the NMDeviceBt to be removed when
a PAN connection's bnepX interface went down in response to a
terminated Bluetooth connection, which of course means that
you can't restart the PAN connection as your phone is no longer
in the NM device list.