@jcbrannan Thanks for taking the time to share this with the community. After a graceful failover, existing flows on the back-up link would continue to used that link until the flow is stopped or no longer exist, while new flows would start going out the active link.
In terms of reporting(This would fall under a feature request), and the best way to see which traffic is going out through the backup WAN link would be to take a packet capture on that link.