Moving an interface outside of the main netns does not deactivate the device unit

[dechamps]

systemd version the issue has been seen with
242-7

Used distribution
Debian Unstable

Expected behaviour you didn't see
If I move a network interface out of the main network namespace, the device unit for that network interface should be deactivated, since systemd cannot manage it anymore.

(Alternatively, I would prefer if systemd could keep managing it and keep bound units alive, but from the discussion on #3939 I get the impression that's not how it's supposed to work.)

Unexpected behaviour you saw
If I move a network interface out of the main network namespace, the device unit for that network interface stays active. However, units bound to the device unit do get stopped, which is confusing.

Steps to reproduce the problem

test-device-ns.target:

[Unit]
BindsTo=sys-subsystem-net-devices-testiface.device
After=sys-subsystem-net-devices-testiface.device

Run:

# ip link add testiface type veth
# systemctl start test-device-netns.target
# ip netns add testns
# ip link set testiface netns testns

Then inspect the state of the sys-subsystem-net-devices-testiface.device and test-device-ns.target. The latter is stopped (which is the intended behaviour according to #3939), but the former is still active, which doesn't make a ton of sense.