Symptoms

When using a foreach loop to iterate through the items in a document set, only the FileName field of the SingleItemRef seems to work any other fields will throw the error:

Document Set ""FileName]" not found.
 

Diagnoses

This behavior was reproducible and a bug was logged. A possible workaround:

a. Since only the FileName field seems to be returned successfully and without error, we can perhaps use the actual document library SmartObject and call its 'Get Document Metadata' method to get the document's metadata. Both folders and document set will be "treated" as folder, as such for the Folder property we can use 'Folder1DocumentSet1' format and pass in the FileName to retrieve its metadata which would include the necessary metadata fields.
 

Resolution

A bug was logged to be addressed in a future update.