I have found a work around. But I did expect the Field Groups to work (but they do not, not ANY of my custom entities utilize the Field Groups). I even tried setting every possible Field Group to the same settings hoping that would solve it, it did not.
On the DataCard..., first enable 'edit contols', expand the 'More Options', then set the 'Default' to execute a LookUp on the Source Table. This works, but I would also like the ability to 'SortByColumns'.
LookUp('Opportunity Feature Function', PrimaryId = Text(ThisItem.OpportunityFeatureFunctionId_.PrimaryId, "[$-en-US]0"))