As the MDA Objects sit on Amazon Redshift, it would be great if Group By on Number fields would be permitted. The Salesforce limitation does not apply to Redshift. In the Report Builder UI, perhaps check to see whether the object for reporting is MDA or SFDC. If the former, permit group by else not?
This needs to be thought through to cover all use cases of filter, calculated measures and Group by when we allow a number to be grouped by. We may take it up sometime in future based on how volume of ask.