Filtering on individual scorecard metrics is currently only available for the default score field. For example, there is a default "Score" field on features in Aha! and I do have the option to filter on those scorecard metrics in a report.
However, there is no default score field for initiatives. Any score that is calculated there is by way of a custom scorecard field that is leveraging the default scorecard. It is not currently possible to filter on individual metrics for a custom scorecard field. Please add the option to filter by individual custom scorecard metrics.