Wow, great detailed writeup of your issue!
First place to check in my mind would be the Background Tasks for Non-Extracts admin view, if you haven't already. You could correlate the time the email was sent to the date range in the dashboard, and look at the Notes field to match it with the subject.
If you can't find it there, then to me, what you're describing is most likely due to a rogue server still running somewhere. Could be Tableau Server running on someone's personal machine, or perhaps a backup server, or the VM zombified itself back to life. If you can't identify it, I'd get your IT folks to trace down which machine was responsible for sending the mail to the SMTP server. That should pinpoint which host is doing this, even if it doesn't quite get you to why it's doing it.
If you know that the subscription is coming from the mail Server instance you now have (the HA environment) and the data is nowhere to be found in the Admin View showing that it actually was sent, then it's time to talk to Support, in my opinion.
Hope that helps,
I checked the background tasks and nothing shows for theses emails. These emails are sent daily at 6am....so, it should be easy to find because nothing else sends then... but nothing shows as sent at that time in the backgrounder nor in the subscriptions view under 'Tasks".
My initial thought was a rogue server as well.... I asked the vm team and they said there is no way that could occur. (They're the "prove it's not your fault before you ping us" team.)
I'm going down the SMTP road now.... I'll let you know what happens there.
I also opened a case with Tableau support as well. Not sure what else they can find, but always worth a try.
Thanks again for your help!!!!