S3 GetObjectAttributes only returns TotalPartsCount on 'ObjectParts' parameter?

Whenever the GetObjectAttributes API is called (via boto) on a Multipart object with the "ObjectParts" parameter, the response for Objectparts is, according to existing documentation, expected to have MaxParts, TotalPartsCount, a list of Parts, along with PartNumberMarker and NextPartNumberMarker if it's truncated or has a part number specified. However, the response only includes TotalPartsCount for any kind of request, even with PartNumberMarker or MaxParts specified in the request. The <Parts> aren't returned either. Is it something wrong with boto or S3 itself? Thanks!