Okay, this is resolved. As suspected, it was related to the configuration of the VLAN for the OPT1 port.
For anyone up against the same issue, the solution is:
follow the instructions for configuring switch ports with VLANs https://docs.netgate.com/pfsense/en/latest/solutions/netgate-2100/configuring-the-switch-ports.html
set your VLAN to have members "4" (assuming you are using the 4th LAN port as your OPT1 for WAN2), and "5"
this is the critical part: for the VLAN members 4 and 5, you must make "4" untagged and "5" tagged -- see screenshot.
I believe this is because traffic from the VLAN must go to the switch (member 5), but traffic on port 4 (member 4) cannot be tagged since your secondary internet provider is not set up to handle VLAN traffic. I could be wrong here, and welcome any better explanation for this solution.
[image: 1747192363633-vlan-screenshot.png]