No traffic is flowing. I have two WAN interfaces, one is dedicated for my homelab stuff and one is for general users. When I power cycle one of the modems only the WAN that I didn't touch stays up.
After reboot, does traffic still flow to the internet without trying to force a DHCP refresh? Most ISP leases are long enough to cover a CPE restart, so pfSense won't bother refreshing a lease that's more than 50% of the lease time left; it's following RFC.