iText RadioCheckField tooltip issue

Question

I am trying to set tooltip (mouse over info) on Acroforms fields using itext. I have no issues with Textfields, Checkboxes and Listfields. But for some reason the tooltip is not getting shown in case of RadioCheckField. Following is the code snippet

RadioCheckField radio = new RadioCheckField(writer, new Rectangle(x, y - 4, x + 14, y - 14), name, exportValue);
            PdfFormField field = radio.getRadioField();
            field.setUserName(toolTip);
            field.setName(name);
            writer.addAnnotation(field);
            radiogroup.addKid(field);    

Invoking setUserName on PdfFormField sets the /TU flag in PDF. When I examined the generated PDF through Rups, I can see that /TU is set for RadioCheckField kids in the radioGroup. But still no tooltip is shown. For 508 (accessibility) compliance tooltip functionality is a must.

Here is a link to PDF file generated from above code http://www.docdroid.net/14son/output.pdf.html Can anybody provide some information?