On system shutdown, zerotier is stopped after the network and gets
itself into a connection timeout loop. It hits the TimeoutStopUSec= and
is forcibly killed by SIGKILL. Order zerotier after network.target so it
can shutdown gracefully while the network is still up.
From systemd.special(7):
at shutdown, a unit that is ordered after network.target will be stopped
before the network — to whatever level it might be set up then — is shut
down. It is hence useful when writing service files that require network
access on shutdown, which should order themselves after this target, but
not pull it in
