As per the Nutanix Prism Web Console Guide (v7.0), specifically at this page:
Expand Cluster - Node Discovery, it is clearly mentioned that:
"The expand cluster discovery step (finding nodes to add) requires that IPv6 multicast packets are allowed through the physical switch. Therefore, the expand cluster process does not work when IPv6 is disabled in your network." This directly confirms that for automatic discovery of new nodes, IPv6 multicast must be enabled and allowed on the physical switches connecting Nutanix nodes. This is a recent update in the cluster expansion discovery process that replaces the previous use of IPv4 multicast in earlier Nutanix AOS versions.