Group of Button fields treated as a Radio Button group

When creating a group of identical button fields PDFBox reads them as a group of radio-button fields, with each widget as a check-box, which is incorrect.
The main field has the class PDRadioCollection and each kid is a PDCheckbox.

Run the following code on the attached file:
PDDocument doc = PDDocument.load( new File("test.pdf") );
PDAcroForm form = doc.getDocumentCatalog().getAcroForm();
List<PDField> fields = form.getFields();
for (PDField f: fields) {
System.out.println("Name:" + f.getFullyQualifiedName());
System.out.println("Type:" + f.getFieldType());
System.out.println("Class:" + f.getClass());
List<COSObjectable> kids = f.getKids();
if (kids!=null) {
for (COSObjectable c : kids)

{ System.out.println("Kid Class: " + c.getClass()); }

}
}

The results are:
Name:Test
Type:Btn
Class:class org.apache.pdfbox.pdmodel.interactive.form.PDRadioCollection
Kid Class: class org.apache.pdfbox.pdmodel.interactive.form.PDCheckbox
Kid Class: class org.apache.pdfbox.pdmodel.interactive.form.PDCheckbox