Our second-tier supporter told me he has worked on this for a long time. He can see the problem occurs on your staging site. However, he is not able to replicate the issue with a similar setup on a clean test site. Our second-tier supporter needs a copy of your site to continue debugging this issue. Even then, he mentioned that debugging won't be easy because the emails are deferred. I remember you didn't allow us to take a copy of the site. Nevertheless, we need your agreement on this matter if you want us to proceed with debugging this issue. We also need your agreement to install a plugin like Duplicator or All-in-One WP Migration to take a copy of your site.
Do you allow us to install the mentioned plugins and take a copy of your site?
We need to discuss the copy of the site with the customer. We will get back to you as soon as possible.
In the meantime, we're going to put the staging back to the way it was before all the tests. We have some work pending since we didn't have access to all our fonctionnalities.
I'll get back to you as soon as I have more information (probably not before next week).
Thank you