Inbound Connections to OPT1 Fail When WAN Drops

[arobar]

I've seen a few similar posts around here, most are so old that they're locked, and none were resolved. I'm hoping someone might have some ideas to resolve this issue.

We have a dual-WAN/single LAN setup using the load balancer's "failover" configuration. WAN is a PPPoE connection, OPT1 is a static (public/routable) IP. The failover routes over OPT1 first, and then WAN.

When WAN fails, outbound connections work fine and route over OPT1 as expected. But inbound connections to OPT1 seem to be dropped somewhere. Even though the OPT1 connection is up and fully functional, I cannot bring up the pfSense web GUI, or access any resources behind pfSense. As soon as WAN comes up, the pfSense GUI loads fine on the OPT1 IP, and I can access web server, and a terminal server, behind pfSense via the OPT1 IP address.

I put a "allow all, log all" firewall rule on OPT1, and traffic to OPT1 during the outage is visible. It is logged as a pass. And yet nothing actually loads remotely. How can I find out what is happening to the traffic once it is accepted by OPT1? Have I perhaps setup a firewall rule wrong that only comes into plan when WAN fails? Any ideas are greatly appreciated!