Default route failover for dual ISP was not working. The route for the backup ISP was configured on a routing-instance, and it was not leaked into inet.0, causing the route to never become an active route on the inet.0 routing table.
We added command: set routing-instances <routing-instance-name> routing-options interface-routes rib-group inet <rib-group-name>, and this injected the local and direct routes from the custom routing instance into inet.0, resolving our issue.