iTextSharp PdfStamper.PartialFormFlattening flattening only some, not all, of the fields

Question

The code below is correctly assigning the value "foo" to the named field, but the field is not being "flattened". I must be neglecting a step, but I don't know what it is. Please advise. Thanks.

public byte[] FlattenSpecifiedFormFields(byte[] b, List<string> fieldNames2Flatten)
    {
        PdfReader reader = new PdfReader(b);
        using (var ms = new MemoryStream())
        {
            var stamper = new iTextSharp.text.pdf.PdfStamper(reader, ms);               
            foreach (string name in fieldNames2Flatten)
            {

                stamper.AcroFields.SetField(name, "foo");
                stamper.PartialFormFlattening(name);
            }

            stamper.Close();
            return ms.ToArray();
        };
    }

Answer 1

Even when partially flattening a form, the PdfStamper FormFlattening property must be set to true. I.e.:

var stamper = new PdfStamper(reader, ms);
stamper.FormFlattening = true;

foreach (string name in fieldNames2Flatten)
{
    stamper.AcroFields.SetField(name, "foo");
    stamper.PartialFormFlattening(name);
}