Update:
I am unable to reproduce my original problem (Table row-heights kept auto-adjusting) and I am unable to fully remember my original preceding actions leading up to it.
I have looked into raising a bug-report into why the “Adapt text options” are greyed-out (in “Position and Size”) and discovered this had been similarly raised before. That earlier discussion had proposed a fix to be scheduled for version 6.2. I see my Ubuntu 18.04 repositories only offer 6.0. So, a “no fix” for me. LibreOffice 7.2 is now available but they do recommend staying with that supplied (repository) for the Linux version used.
I would accept this thread to be closed.
Many thanks for your time and patience.