Course Completion Certificate Message Not Sending

Summary

When a user completes a course, the Course Completion Certificate Message does not send, though the completion is registered.

Content

We recently completed a system interface facelift moving from a primarily Classic Pages design to the new Learn Center Designer Pages. With release 19C going into production around the same time, I'm not sure if it is the new release or the Classic -> Designer page switch causing the issue below.

Our Course Completion Certificate Message no longer sends to the user's email when they complete a course. The completion is registered correctly and reflected on their transcript (with the certificate available for download from there), just no email is sent.