Major Description
Thanks to mewmewball for catching this one.
Accidentally executing the same instance of Solr twice causes the second start instance to die with an "Address already in use", but not before deleting the first instance's live_node entry, emitting "Found a previous node that still exists while trying to register a new live node <node> - removing existing node to create another".
The second start instance dies and its ephemeral node is then removed, causing /live_nodes/<node> to be empty since the first start instance's live_node was deleted by the second.