Why does quantity shipped not updated when partial quantity available at order entry

Hi,

When enter order quantity more than available quantity in order detail line, shipped quantity (SOQS) field is not getting updated with partial quantity available for the item instead entire order quantity is updating in backorder field (SOBK) at sales order detail line with status 900/540 at order entry though below checks are enabled.

1. Availability check is enabled in Sales order version processing options.

2. Availability check and backorder allowed enabled in item master, item branch, branch plant constants and customer master billing instructions.

3. Partial shipment and partial order shipment also enabled in customer master.

We would like to have a split line for shippable quantity and backorder quantity when partial quantity available at order entry.