SQLXMLBULKLOAD Not Inserting Null Values

i have attached XML and XSD file
i bulkload xml file into 2 tables .ManifestID is the Relationship between 2 tables

CREATE TABLE [dbo].[BIOS_DataLoader_LDR_Manifest_Detail] (
[manifest_id] [varchar] (20) COLLATE SQL_Latin1_General_CP1_CI_AS NULL ,
[order_num] [bigint] NULL ,
[track_code] [varchar] (20) COLLATE SQL_Latin1_General_CP1_CI_AS NULL ,
[box_id] [varchar] (50) COLLATE SQL_Latin1_General_CP1_CI_AS NULL ,
[box_type] [varchar] (20) COLLATE SQL_Latin1_General_CP1_CI_AS NULL ,
[scan_time] [datetime] NULL ,
[scac_code] [varchar] (20) COLLATE SQL_Latin1_General_CP1_CI_AS NULL
) ON [PRIMARY]
GO

CREATE TABLE [dbo].[BIOS_DataLoader_LDR_Manifest_Header] (
[trailer_id] [varchar] (100) COLLATE SQL_Latin1_General_CP1_CI_AS NULL ,
[trailer_close_date] [datetime] NULL ,
[manifest_id] [varchar] (20) COLLATE SQL_Latin1_General_CP1_CI_AS NULL ,
[manifest_qty] [int] NULL ,
[origin_facility] [varchar] (20) COLLATE SQL_Latin1_General_CP1_CI_AS NULL ,
[destination_facility] [varchar] (20) COLLATE SQL_Latin1_General_CP1_CI_AS NULL
) ON [PRIMARY]
GO

When the first TrackCodeid is NULL
all remaining NULL TrackCodeid is entered as null in the database table
but if the first/prev trackcodeID is not NULL then following null trackCODEID is populated with the prev trackcodeID and not as null in database
<TrackCodeId>ABCDEFG</TrackCodeId>
All Null TrackCOdeID is populated as ABCDEFG
if we remove ABCDEFG and then bulkload all null values are populated
if a null trackCODEID is to be inserted the prev trackCODEID must be null and must not contain any value