I went through this guide (https://docs.opnsense.org/manual/how-tos/multiwan.html) and set up multiwan here. I was testing with uPNP and noticed that not all my firewalls would respond to uPNP requests, even though all had the plugin installed, configured similarly, etc.
I narrowed it down to the firewall with multiwan enabled. I went into the firewall Rules on the inbound interface (LAN side) and modifed the Gateway part of the internet access rule to no longer use the WAN_FAILOVER group. Just the default gateway setup. uPNP started working. Went back to WAN_FAILOVER, uPNP no longer functions.
Not sure how to submit this as a bug, or if anyone would pay attention, but the uPNP configuration only specifies one of my two internet links (my primary one) - if that matters.