The command you're using is correct, but it seems the dashboards we have bundled with Beats are pre-processed by the Beat binary.
To import:
curl -s -XPOST https://<kibana:port>/api/kibana/dashboards/import -d "@dashboard.json" -u elastic:password -Hkbn-xsrf:true -Hcontent-type:application/json
To export:
curl -s https://<kibana:port>/api/kibana/dashboards/export?dashboard=<dashboard-id> -u elastic:password -o dashboard.json