If you use the RVC tool to add a host to a stretched cluster, the configuration of the new host is incomplete.
After you use the RVC tool to add a host to a stretched cluster, on the Summary tab for the cluster the Configuration Status for vSAN appears as Unicast agent unset on host.
When a new host joins a stretched cluster, vSAN must update the configuration on all hosts in the cluster. If you use the RVC tool to add the host, the update does not occur. The new host successfully joins the cluster, but its configuration is incomplete.
Verify that all hosts are connected to vCenter Server, and click the link provided in the Configuration Status message to update the configuration of the new host.