Losing rounded corners when flatten PDF (dynamic convert to standard)

We want to display and print XFA PDF documents created with Adobe Lifecycle, within a viewer.
Therefore we flatten XFA PDF files as described in the documentation (change doctype dynamic to standard)

Using document As Pdf.Document = New Pdf.Document(New System.IO.MemoryStream(source_data))
If document.Form.Type = Pdf.InteractiveFeatures.Forms.FormType.Dynamic Then document.Form.Type = Pdf.InteractiveFeatures.Forms.FormType.Standard
document.Save(destination_file)
End Using

Now the problem. When we have an existing PDF with rounded corners somewhere in the document, after flattening these rounded corners are disappeared.

See the attachements.
XFA with rounded corners (upper right corner)
Result without the rounded corners.

When i do the same flattening from dynamic to static instead of standard, the rounded corners still exist, but not all the XFA data is removed, so it is not fully flattened.

Also printing of these files with Aspose.Pdf.Facades.PdfViewer has the same problem. When printing the file the rounded corners are lost.
We use version V10.1.0.0 library Aspose.Pdf