Create a new Standard Sales Order
Add 1 Sales Order Line with quantity of 1
Prepare the order. Sales Order Line 1 should have quantity reserved of 1
Add another sales order line with quantity of 1
Note that the newly added line will have quantity reserved of 0.
The test case above leave you with a sales order document in an inconsistent state - effectively a Prepared Order that's not fully prepare.
1. Move reservation of stock to "Complete".
2. When order is with Prepared status, auto prepare the order or just the newly added order line.