OpenVPN client tun adapter loses its IP address on network restart

This caused by referencing the tun1 device in /etc/config/network with option proto none - it will cause netifd to clear all settings on this device when network is restarted.

Best remove the dummy interface declaration to prevent netifd from interfering with openvpn. You can use option device tun1 in /etc/config/firewall instead if you want to assign it to a zone.