Grafana strange column title after json aprsing

Question

I used the promtail config file to parse a json file and load the fields as labels into grafana:

example of log line:

{"time": 1647531679.5431354, "location": "Milano", "serialNumber": "hype3", "hostname": "hype3-hostname", "vmName": "vm9", "status": "WARNING"}

promtail.cfg:

....
- job_name: backup-status
  static_configs:
  - targets:
      - localhost
    labels:
      __path__: /var/log/backup/jobs.log
  pipeline_stages:
    - json:
        expressions:
          time: time
          vmName: vmName
          status: status
          hostname: hostname
          location: location
    - timestamp:
        format: Unix
        source: time
    - labels:
        vmName:
        status:
        time:
        hostname:
        location:
    - output:
        source: status
....

The labels load correctly in Grafan:

But when I go and try to create a plot with this data source the table view looks like this:

I was expecting to see as column names the labels of the log lines. Did anyone encounter a similar problem?