Anonymous , I think I found where the issue came in, and so was able to resolve, but this might be a bug in how dataflows are handled. This was an existing dataflow imported into a dataset. The dataflow query is a select * from a datedim SQLview, we use that as by definition we want to bring in all view fields. MonYY was added to the view, but does not show up in the dataflow by default, you edit the query, save and refesh and it was brought in. You then have to edit your dataset query of the dataflow, and it brings in the MonYY, but although it is imported as a Text field it has this odd non grouping behaviour. The solution was to delete the table and bring it in from a new Get Data and reapply any relationships and formatting. I think this is probably reproducable.