Similar issue here. I set the custom cert to email me when the certificate is issued. The certs I receive by email do not correspond to the ones listed on /mod/customcert/view.php, nor to the activity restriction set.
I rec'd by email three certs; two users who met the activity restriction and one who did not. On the day that I rec'd those certs, the page continued to indicate "Recipients: 0". Now, several days later, it lists "Recipients: 2" and shows the two that DO meet the activity completion restriction, but not unexpected one that was sent by email.
BTW when I first tested this module in mid-2018, I set it to email the recipients. It promptly sent out 50 emails with PDF attachments--some to the correct students, some to those who did not meet the activity completion requirement, and some to users who were not even enrolled in the course.