pfSense

"propose you add an option to keep retrying DHCP renews on the WAN when the gateway becomes inaccessible." - Based on what you've said, the gateway doesn't become inaccessible. That's why it's broken.

Have you tried setting the monitor IP (static routes, WAN interface) to something out on the Internet?
I suspect it's just set to ping your cable modem.

This is an inherent problem with DHCP leases, there's no state, so there's no (easy) way to know when it's broken/changed.

I don't think this is a pfSense bug - from the pfSense manual:

"Alternative Monitor IP: An address to ping via this gateway instead of the gateway itself. If the gateway is local, such as one directly on a CPE or modem, then pinging a remote address such as a DNS server is a better measure of how useful or alive the WAN may be."

the cable modem (Arris/Moto SB6183) has no ip, its a bridge, the gateway is somewhere at my ISP. the modem stays alive, but the gateway goes unreachable when there's a cable system outage. we've had two this winter, one was about 8 hours, the 2nd outage last night was about 1 hour. changing the monitor address would not help this, afaik, it detected gateway down, and the WAN dhcp timed out, then it retried however many times and quit trying.