Truly baffling that this wasn't implemented immediately as part of the grouping capability, no less that it still hasn't been so many years later... it's one of the single most basic, foundational features that any user would need when adding custom groups. The workaround of creating a separate mapping table and adding a calculated field value for every single record in the dataset is one of the most egregious examples of adding unnecessary complexity and creating redundant data for such a basic and simple exercise that I've ever encountered. Excel pivot tables have been able to do this for at least a decade, if not longer (not to mention in a much more convenient way by clicking & dragging fields right on the table/visual itself, another area on the list of features in which PowerBI is many years behind Excel in... but I suppose that's beside the point for this particular issue).