Dual WAN failover due to DNS failure , possible?

[jradxl]

2.4.3-RELEASE-p1 (amd64)
I have a Loadbalance and WAN failure configuration using Cable (on WAN) and VDSL ( on OPT1) providers, where their routers are set to modem mode.
The Monitor IPs are of course IP numbers, for example 8.8.8.8
Last night, the Cable modem lost it's ability to process DNS requests.
Diagnostic Ping to 8.8.8.8 worked on both WAN and OPT1, but pinging cix.co.uk only worked on OPT1.
Clearly the Failover didn't switch.
I rebooted the Cable modem and my system recovered.

Can I protect against this happening again?
Should it have happened - should pfSense have obtained the DNS from other DNS servers I've configured?