If you run it through the veraPDF Docker and select the PDF/UA-1 conformance check, it notes that the File attachment annotation is not contained inside an Annot tag: "An annotation, excluding annotations of subtype Widget, PrinterMark or Link, shall be nested within an Annot tag"
From https://github.com/metanorma/mn2pdf/issues/267#issuecomment-2376348257: