Scrap in challan

Hi,

We have issue like as below.

We are sending assembly component to the vendor he will assemble the component and sends back single material as semi finished

Process flow is Create Sub con PO> Issue material against Sub con PO> Create challan> Receipt material> Reconcile challan>

But challan shows incomplete as child componenet shows as scrap generated which is showing as due for reversal

Why scrap is showing in challan how it avoided.

Regards