I haven’t had time to test the theory yet,
But I think (in 7.0) the groupby term doesn’t take the query itself into account. Which results in groups without data also being visible.
You can toggle these empty values with the switches in the “Legend” panel of the plug-in.
A similar item was posted here: Grafana 7.0 Query Result different with Kibana result in Lucene query format
Although it shows the items, data for the items is filtered out