When you filter with the field 'Assigned Source of Supply' , 'Yes' or 'No' , the result is not clear.
Reproducing the Issue
App Manage Purchase Requisition
The Filter 'Assigned Source of Supply' will show those PRs with a vendor assigned as: Scheduling agreement, Contract, Inforecord, or Fixed Vendor.
The PRs with a Desired Vendor are not considered as Assigned.
The Purchase Requisitions with a Desired Vendor will show the supplier code in the column 'Assigned Supplier'.
KBA , MM-FIO-PUR-REQ-SOC , Fiori UI for Sourcing Purchase Requisitions , Problem