@stephenw10 I've just encountered the same issue as reported in the example you give (https://redmine.pfsense.org/issues/13224) where I received an alert from NUT that my UPS was not responding and then a barrage of 'is available now...' notifications for my secondary WAN connection.
There was also a second notification from NUT when network connectivity was re-established to the UPS (only two notifications from NUT, the down notification and the up notification).
I wasn't at liberty to take any action that might cause network downtime (reboot, etc...), but clearing out the repeating notification from /var/db/notifyqueue.messages worked to put a stop to it. Not sure what, if anything, NUT has to do with it, but it seemed interesting that the circumstances I encountered matched what was reported there.