rare recurring error: CiviMail will not send an empty mail body, Skipping
It appears in a tiny fraction of mailing recipients (less than 0.01%), we are not sending the mailing because contact details are empty. The contact details contains custom tokens that we have added, but no actual core contact data, e.g. preferred_mail_format (which is required to compose a message). To be clear, no contact data is present, it's not that preferred_mail_format is NULL.
Perhaps this is caused by a failed db query somewhere along the way. Will require further debugging to get to the bottom of it.