Comment # 5 on bug 940522 from Marius Tomaschewski

(In reply to Dr. Werner Fink from comment #4)
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=779902

Yes, that's it. openvpn@.service is using PrivateTmp=true:

# cat /usr/lib/systemd/system/openvpn@.service 
[Unit]
Description=OpenVPN tunneling daemon instance using /etc/openvpn/%I.conf
After=network.target
PartOf=openvpn.target

[Service]
Type=forking
PrivateTmp=true
PIDFile=/var/run/openvpn/%i.pid
ExecStart=/usr/sbin/openvpn --daemon --suppress-timestamps --writepid
/var/run/openvpn/%i.pid --cd /etc/openvpn/ --config %i.conf
ExecReload=/sbin/killproc -p /var/run/openvpn/%i.pid -HUP /usr/sbin/openvpn

[Install]
WantedBy=multi-user.target openvpn.target