Lot Number Item Tracking Quantity in Production Order does not update the same back to Sales Order

Hi,

I need an advise here.

My scenario as below:
To manufacture a same product, the business may use different BOM No. and routing (the main component will change, and hence the routing with change accordingly). The Finished Goods is set to Lot Number Tracking.

The process is:
1- Create a Sales Order with FG101 of Quantity 5
2- Create a Released Production Order (RPO) from Sales Order
3- Change the RPO > Line, the default BOM & Routing & Quantity become:
line 1000 - FG101, BOM101, Routing 101 , Quantity 4
line 2000 - FG101, BOM102, Routing 102, Quantity 1
4 - Refresh the Production Order for "Routing" & "Component"
5 - Reserve Line 2000 back to Sales Order (line 1000 > Reservation line still remain, but quantity has changed to 4, which is OK)
6- When check back to Sales Order, the total quantity reserved is 5
7- post the Production Output Journal & Consumption
8- Complete the RPO and change the status to Finished Production Order.

Result:
- check the Sales Order > Reservation Entries still 5
- check the Sales Order > Item Tracking Line --> system only tag the Lot Number for Quantity 4 only, the other 1 quantity is not 'auto' track and tag to Item Tracking line.

Question:
May I know did I miss out any steps or setup? Or any better suggestion?

Many thanks in advance