>I have had this configured to IP passthrough mode without issue for years. But, after the Opnsense upgrade (and defaults), I did notice that my gateways were configured differently. Previously, my upstream WAN gateway was the IP address of the BGW320-505 box. Now, my upstream WAN gateway is my WAN IP address with a .1 substituted for the final digit.
This is critical info. You have been configured for IP Passthrough for exactly however long ago you updated.