SUMMARY
see $SUBJECT

STEPS TO REPRODUCE
1. open a PDF
2. add a stamp (custom or provided by Okular)
3. open print preview, or export to another PDF

OBSERVED RESULT
stamps are not visible in the resulting print (preview)

EXPECTED RESULT
stamps should be visible.
I can accept that the *editable* stamps in the PDF file are only visibly in Okular. But if I print a PDF (even into another PDF) the stamps should become a fixed integral rasterized (or rendered) part of the PDF file, and be visible and printable by any other PDF application (but possibly not editable any more).

I am using the stamp feature to add my handwritten signature to PDF files just like I used to do in OS X Preview (it works beautifully there), and this works fine - up to the point where I want to "print to PDF" to give the PDF to somebody else.
 

SOFTWARE/OS VERSIONS:
KDE Neon 20.04 LTS with all updates applied.