Rancher 2.5.2 - Some clusters don't receive fleet agent

I noticed some clusters did not receive the fleet-agent and the Clusters view in Continuous Delivery in the Cluster Explorer UI shows them with “Waitcheckin” in red.

Clicking into one of the clusters that are in this state with no fleet-system namespace or anything, I see “secrets “c-tbrth-kubeconfig” not found” at the top of the Explorer page for that cluster in the Continuous Delivery view.