Hi Steve,
After you mentioned, I checked Control Tower Planning Area copied from SAP5 and also Inventory Planning Area copied from SAP3. In both PERIODID is marked as Root attribute for different Planning Levels while S&OP Planning Area copied from SAP2 has Monthly as Root attribute.
While I am not sure if this is intentional or by design but both custom (copied from SAP standard) Control Tower and Inventory Planning Areas with PERIODID as Root attribute works ok with keyfigure data loaded.
Regards,
Somnath