thanks for the tip.
This little fix works as you describe.
I'll have wait to tomorrow to see what happens with the rest of the cron tasks as well.
Update - June 2020.
Since May (with no major upgrades in between - I think journal was updated April 26 ) this no longer works.
Have now done a full upgrade to 3.8.3 and tried it again - no luck.
I have looked at Michael's link : https://github.com/elearningsoftware/moodle-mod_journal/issues/47, but the code has changed in the mod/journal/privacy/classes/provider.php since his post.
It looks like somethings is broken in the mod/journal - or I have some inconsistent data which kills it.