Why "Download Document IDs" web service task isn't responding with document IDs even though they are available in procurement contracting workspace and could be easily downloaded with SOAP API "Download Workspace Documents"

The Download Document IDs web service is supported only for order form documents.

Order Forms
You can create order form documents in sales contract requests and workspaces only.
If you create a sales contract workspace from a sales contract request, all Order Form documents are copied to the sales contract workspace.

Documentation help : https://help.sap.com/viewer/eb12a8256a244b86be23c9b4e4e751da/cloud/en-US/7c94d50171ea1014a645cc284c80f13d.html

Strategic Sourcing