If you assign an OpenVPN interface, *never* give it an IP. Set it to an IP type of "none".
You can't add a gateway for policy routing to an OpenVPN server that way. A client gets an automatic gateway, not sure we we don't do the same for a server if it's shared key or a /30 though.
If you need to route via OpenVPN, add a route statement into OpenVPN's config, don't rely on system gateways.