Send status indicates the newsletter sending has been completed. However, the number of newsletters sent differs by x hundreds from the total number of newsletter subscribers.

The Simplenews conjob is triggered every 15 Minutes. It is controlled by Ultimate cron. The newsletters are sent out in packets of 50. After the mailing is completed, there is a difference between the number of newsletters sent and the total number of newsletter subscribers. The difference has a scale of several hundred (with a total of about 2800 newsletter recipients). The System Log does not contain any error messages. However, some completion messages of the cronjob are missing, which numerically correspond to the difference.

Now I don't know if all recipients have been served or if there are incidents in the shipment that result in incorrect logging. How can I debug this?

I use Drupal Version 8.9.13. Ultimate Cron Version: 8.x-2.0-alpha5