I had to add the relabel_configs under both the node_exporter and windows_exporter jobs as follows:

relabel_configs:
  - source_labels: [__address__]
    target_label: instance
  - source_labels: [instance]
    target_label: hostname
    regex: '([^:]+).*'
    replacement: '${1}'

I had missed the above under the Windows_exporter job previously.

Cheers.