Add field from calculation (Table Panel, ES datasource)

I’m trying to add a field from a calculation in a Table Panel from an ElasticSearch data source with a Raw Data query.

However, as seen on the screenshot, I can’t select any fields.
This also happens without any other transformations before it:

Am I doing anything wrong/not understanding the feature or is this a bug?
(Grafana 7.3.4)

I just found out that there is an open github issue for this: