Data of pivot table with external source is imported as string items. When grouping is done in `ScDPSaveGroupDimension::AddToCache`, `ScDPSaveGroupItem::ConvertElementsToItems` converts string numbers to double items, but the source contains string item for the same data. Because of this mismatch grouping is not preserved. bug-document: forum-mso-en4-58929.xls Change-Id: If32311204eca4009ec2a00e37271f7bd5cc2d45c Reviewed-on: https://gerrit.libreoffice.org/c/core/+/202734 Reviewed-by: Balazs Varga <balazs.varga@collabora.com> Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com> Signed-off-by: Xisco Fauli <xiscofauli@libreoffice.org> Reviewed-on: https://gerrit.libreoffice.org/c/core/+/204229 Tested-by: Jenkins