Hello @sean-h,
The returned warning is not expected. I have performed several tests on test installations and there was no such warning message. Please note that this is not a fatal PHP error and if you have confirmed that the Cron is executed correctly, you can safely disregard the message.
The issue could be generated by another plugin’s script execution triggered by the Cron. You can disable all plugins and test again. Also, you can check the wp-config.php file for any unused directives.
If you are triggering the wp-cron execution via curl/wget or even by directly executing the file with PHP-CLI, I would highly recommend you to consider using WP-CLI. The required command you can find in the article below.
https://eu.siteground.com/tutorials/wordpress/real-cron-job/
Once you test the execution, you can define the Cron with the output redirect to /dev/null in order to avoid the automatic notification mails sent by the Cron service as the WP-CLI always returns output.
Best Regards,
Dimitar