I grateful for the swift responses and I understand everything written; however the behaviour I am seeing is as written in my heading. Things were more stable before I updated to 6.4 (from 6.0 I think).
I use a MacBook Air, recently updated to the latest Operating System (10.15 - Catalina). On a Mac the settings you mention are in
Preferences rather than
Tools -> Options. I attach the relevant info and I can assure you that neither Default Text not Table Contents Styles use the Liberation Sans font. Yet this is what I get.
Worse: almost any formatting in a table changes when I press tab into a new row. Heading rows change etc.
One thing I wonder is whether I need to tweak Java. I don’t currently ask LibreOffice to run anything. My understanding is that the Mac setup requires JDK not JRE but I also recall a few years ago some confusion around these settings.