The default namespace Uri (index 1) of an OPC UA server is different for each device. If nodes are in this namespace, the Uri is not known during offline engineering. Part 83 describes the following in “A.9 Instance Mapping”:
"In the InstanceHierarchy, an empty or missing NamespaceURI (of the BrowseName) or UaNodeNamespaceUri shall imply the default NamespaceURI of the OPC UA Server (i.e. Namespace index 1)."

This means that in this case the descriptor will not contain a NamespaceUri. As the descriptor does not contain any information about the NamespaceUri, the resulting ConnectionConfigurationSet created by the tooling will not contain the NamespaceUri either.

In Part81 the chapters “13.3.2 PortableNodeId provided” and “13.3.3 PortableRelativePath provided” describe how to resolve the NamespaceUri of a PortableNodeId/PortableRelativePath to the namespace index in the server. Both chapters should be extended to cover the case where the NamespaceUri is empty. If the NamespaceUri is empty, it shall be resolved to the namespace index1.