I guess you are using policy-routing rules on your LAN, to direct traffic to WAN1 and WAN2 according to your failover and load-balancing needs.
In that case, you need to have a rule on LAN that matches source LANnet, destination OpenVPN tunnel subnet (10.0.8.0/24), gateway none. That will allow the traffic returning from LAN to the OpenVPN client to be passed normally to the routing table, which knows how to route it to across the OpenVPN tunnel to the client.
Without that, the traffic can be forced out WAN1 or WAN2 by a policy-routing rule, and of course never reaches the OpenVPN client.