Just an update.

I can successfully get through the booking process if I book a ticket just for myself. The error occurs when I attempt to add one or more additional passengers to the itinerary (which I kinda need to do since it is a family of four and I want us all to get the 100K benefit).

The fact I can book a ticket for myself with the auto-populated passenger data, including the FF#, leads me to conclude that the actual number itself is correct and the error is something wrong in their backend code.