I generally have the "view non-printing characters" option turned on in the edit menu. Under "Tools --> Options --> LibreOffice Writer --> Formatting Aids --> Display of," I have all non-printing characters checked.

When I export to PDF, all non-printing characters disappear in the edit window, even though the "view non-printing characters" toggle remains on. This is because the first six of the "Display of" tickboxes have been cleared by the PDF export. I have to manually re-check all the boxes in order to see the non-printing characters again.

The entries cleared are: Paragraph end, Custom hyphens, Spaces, Non-breaking spaces, Tabs, and Breaks.

This is 100% repeatable and occurs every time I do a PDF export.

My PDF export options under the "General" section are all turned off because of the "PDF/A-1a" bug that prevents PDFs generated by LibreOffice from opening in Adobe Acrobat (invalid color space). The plain vanilla PDFs open fine.