Hi Peter,
At present, we have a support to fill and flattening the static XFA fields and we do not have support for filling and flattening dynamic fields. We do not have any plans to implement this feature due to XFA has been deprecated in PDF 2.0. We are not preserving Adobe Usage rights (UR3) during the XFA form filling process instead we removed this permission from the document.
Kindly refer the following link for more information,
https://help.syncfusion.com/file-formats/pdf/working-with-xfa#filling-form-fields-in-an-existing-xfa-document
Please let us know if you need any assistance in this.
Regards,
Irfana J.