In my case, OpenVPN fails to start automatically due to the absence of /run/... directory. I tried to create it automatically by means of adding mkdir -p /run/openvpn to /etc/rc.local, but the launch of OpenVPN client appears to happen before the mkdir. So, it does not help. Having said that I am able to successfully launch the client manually once logged in.
I expected to use systemd's Restart=on-failure, but I fail to find a corresponding file.
$ dpkg -l | grep openvpnii openvpn 2.3.10-1ubuntu2.2 arm64 virtual private network daemon