The label filter doesn't work on the Alerting page. When entering the label to filter by the URL parameters aren't updated and nothing happens on the page. The page also doesn't filter by labels when you navigate directly to the page with a URL that has label filter parameters.   

4.19

1. Navigate to Observe > Alerting
2. Change the filter from Name to Label
3. Enter a label that exists on at least one alert
    

Nothing happens

The URL parameters are updated and the list is updated properly.