I am not sure, if this is a bug or a feature request.
It seems to be point of definition what Passthrough means. For me Passthrough means: Do not touch the content of the PDF for display and export. Right now this is not true. When a PDF is placed having a form element, the form element will be shown after import. Exporting this document - regardless the PDF setting - the output will be cleared from the form element. I do not expect, that the form functionality is preserved (because the Affinities have no PDF form exporter), but the layout.
Attached is a simple PDF with a checkbox.
checkbox.pdf