You are confusing two different Concepts:

Assembly item is an Inventory item. That means the value is in Asset. When you Build it from components on hand, which are in Asset, you didn't Change Anything financially. You changed Stuff into different Stuff. That's why there is no Job Tracking this. This is Inventory = Inventory. I take 4 legs, a Back and a Base and make a Chair.

Later, I sell the chairs. Now this is the Financial transaction that has to do with Customers and Sales and invoices or sales receipts. Or, I quoted that I would completely outfit your Dining room for $xx and you signed the Fixed Rate Contract. So, once I know we have made all the chairs and the table, I would Charge you the Contract fee, and use Adjust, to Relieve from stock the chairs and that table and Job Track that these were not Sold to you as Details, but Removed to fulfill your Contract Bid Fee.

You don't Job Track the Build, because you needed to make the products available First, before you can even have a Sale.

You can Justify the Build, by showing evidence that a Customer Order exists. And think about this: there might be 5 customers needing 100 each of that Product. That's why the Build is for 500; the Sales Order shows each of the 5 customers, their info, required delivery date, etc.

You are trying to attach two Steps into One, but they exist separately. One is not Changing the financials; Build Assembly is not a Customer Activity. It is an Operational Activity to provide what is needed for Customers, as the Fulfillment step, which is Later.

And I showed Job Reporting will include the cost from the Build; because that is the Basis on Hands of the item Built, and then Sold to the customer. It doesn't matter if you sold it or adjusted it to relieve it from stock; that Movement carries Basis or Cost with it. That is reportable, because the Sale or the Adjustment is where you Job Tracked it.