systemd unit file should explicitly pass --RunAsDaemon 0
The current systemd unit uses "Type = simple", so systemd does not expect tor to fork. If the user has "RunAsDaemon 1" in their torrc, then things won't work as expected. This is e.g. the case on Debian (and derivatives), since there we pass "--defaults-torrc /usr/share/tor/tor-service-defaults-torrc" (that contains "RunAsDaemon 1") by default.
The only solution I can see to this problem is to explicitly pass "--RunAsDaemon 0" when starting tor from the systemd unit file.