It's confusing that netpols.yaml is JSON

[esnible]

netpols.yaml is JSON, which surprised me.

If YAML is intended,

jq '.items[]' < netpols.yml > /tmp/netpols.json 
kubectl apply --dry-run=client -f /tmp/netpols.json -o yaml | yq ".items[]" > /tmp/netpols.yaml

... will produce it