Not sure if this helps, but - I remember something similar, but it involved dynamic PDF documents, and I seem to remember it was fixed with the service pack for ES (that would be the .1 of 8.2.1).
The problem was that the data was added to the PDF, but it wasn't rendered into the "visual layer" until the document was opened. This was because of the "render at client" option that is automatically on with dynamic PDFs. When the NOXFA tag was run, the data was thrown away and therefore never made it to the rendered version.
The solution was to make sure the template was static and the render at client option was off.
If you look at the document after it comes from Forms, do you see the data?
Well it did not work, we have updated the version from the ES 8.2.1 to 126.96.36.199 and still IS running incorrectly.
We used two different mechanisms to place XML data in PDF:
None works, the imported data do not appear in the final flatten PDF.
Where it works properly is with LC ES2, we have to update, because we rush to deliver this development.
While we understand that should work with the ES version 8.2.x, all documentation is in your website says this, but the reality is that (at least to us) does not work, the static PDF preloaded (with DataIntegration or Forms) and then flattened with Assembler NOT show the latest data preloaded.