If you left backup filenames to defaults, the .mbz files would have a date/time stamp in the filename.
Something like the bold part below:
backup-moodle2-course-136-mdlsampler-20200302-1908.mbz
When you attempt to restore, what is the date/time stamp of the .mbz you see?
While you might have auto-backups settings the way you want, the creation of those is depended upon the cron job for your site.
Is cron job running as it should?
Also, if you set the path to php-cli in paths setting for your site, there will be a 'run now' link in the row for the automated backups scheduled task and in column for 'last run':
/admin/tool/task/scheduledtasks.php
Could see if that works. Caution however, if you have a bunch of courses and they are large, 'run now' could take a long time or even time out.
'SoS', Ken