Best solution/workaround at present seems to be this…
Go to LUBUNTU MENU: Preferences -> LXQt settings -> Session settings -> Environment variables
Then, add this:
Variable Name: SAL_VCL_QT5_USE_CAIRO
Fill the Value with: true
Restart your computer, and it will resolve the fonts rendering while exporting documents to a PDF on LibreOffice.
(There is just one small inconvenience – see below).
A few other people have hit this too, however it has been reported and discussed elsewhere as problem with PRINTING from LibreOffice (see “Printing problem with 20.04” in this forum). The connection between these two manifestations of a problem is that LibreOffice generates a PDF, which is then sent to be printed via CUPS. Without the above (or similar) fix, the generated PDF is invalid, because the required fonts fail to get embedded.
A MINOR INCONVENIENCE : with this “fix” in place, you will always have to start either LibreOffice-writer, LibreOffice-calc, -draw, -impress, etc, because trying to start “bare” LibreOffice will throw an error. In my opinion this is a minor inconvenience compared to not being able to export to PDF, or print anything at all.