Group by not Working as expected

I try to group my photovoltaik-data per month to display it in a bar chart. The data is stored in two influx-DBs (Version 1.8.x).
With the Grafana transformation, I already could create a Timeseries, that looks pretty good and only must be grouped by Month.

If I enable the Group by -Transformation, it will result in no data, and the list of fields is changed

Any ideas?

Kind regards Rainer

what happens if you disable your first join transformation?

Because the data is out of two queries, this leads to two different resultsets (may this is not the correct term).

And this results in “No data”