In order to stymie what I anticipate might be the next offerings in this matter ...
Is it at all possible to amend the coding in the test email you created so that there are line numbers?
That might make it easier for me to craft some future response to them by saying ”x happens on line y on interacting with your service, and by the time we get to line z the incorrect default behaviour w has been implemented”
If that is too much hard work, i will understand. Just trying to remove a further factor they could try to use to weasel out of correcting this issue.