cron jobs stopped working under Joomla 3.9.13 and CiviCRM 5.18.4
I had difficulties getting cron.php to log progress and, as the issue was on a remote server would have difficulties debugging.  As a consequence, I rolled the CiviCRM installation and database back to backups taken before the issue occurred, and then reinstalled the CiviCRM upgrades.  The cron job now appears to be working.  Fortunately, the problem was on a test system so it didn't matter about any recent work.  Had it been on a live system, it the issue would have been much more serious.