I wonder if this problem could be linked to an old variable font compatibility issue with LibreOffice (Fonts not remaining bold when exported to PDF or Printed)
How do you expect to be helped? There is no information in your question (well, it is a mer complaint). Please, edit it (don’t use a comment, modify it) to provide minimal information: OS name, LO version, save format, name of font.
For best chance of success, attach a 1-page sample file.