Ahh, the rolling enrollment or validity periods makes it a bit funny.
And to be clear, so that I don't give you false hope: the Custom certificate plugin handles the customcert_issues table in the same way (it does not delete records when a user is unenrolled), so I think you'd be in the same situation even if your client changed. Various plugins seem to handle this in different ways, even among core activities (Forums, Assignments, etc.). Must depend on a mixture of pedagogical principles, and preferences of the developer.
In a way, it sounds like the certificate is a proxy for performance on some other Moodle activity, so perhaps using that data directly would be best for your exact situation. Students could still get the cosmetic certificate that has a new date every year. To validate the new yearly test completion, an admin could just look at the activity page or gradebook directly (probably tedious). Or maybe a clever query in the Ad-hoc database queries or Configurable reports plugins would fulfill the role of the "Date received" column for the certificate. But now I am adding more plugins to your client's site, which only makes your job harder.
And to be clear, so that I don't give you false hope: the Custom certificate plugin handles the customcert_issues table in the same way (it does not delete records when a user is unenrolled), so I think you'd be in the same situation even if your client changed. Various plugins seem to handle this in different ways, even among core activities (Forums, Assignments, etc.). Must depend on a mixture of pedagogical principles, and preferences of the developer.
In a way, it sounds like the certificate is a proxy for performance on some other Moodle activity, so perhaps using that data directly would be best for your exact situation. Students could still get the cosmetic certificate that has a new date every year. To validate the new yearly test completion, an admin could just look at the activity page or gradebook directly (probably tedious). Or maybe a clever query in the Ad-hoc database queries or Configurable reports plugins would fulfill the role of the "Date received" column for the certificate. But now I am adding more plugins to your client's site, which only makes your job harder.
I am constantly finding the balance between "doing things the way I want" and "doing things the way Moodle will allow". Good luck!