Rescheduling the date of a sales order is not working as expected

Summary:

I am testing an operation to change the scheduled ship date via SP for an order that has failed to reserve inventory since out of stock.
However, I am not getting the expected results after executing steps 1-6 below.
How I can get the expected results?

Content (please ensure you mask any confidential information):

1. SP collects data on orders
2. SP executes the plan
3. Manually create a plan order on a day prior to the day the sales order is generated.
4. Re-execute the plan.
5. Link the planned order to the GOP. (Automatic)
6. Reschedule the order on the OM side.

Expected Result : The scheduled ship date of the order is rescheduled to the suggested due date of the manually created planned order.