Can no longer make one-off edits to an Advanced Mail email sent to one Contact

Using the Advanced Mail email editor, how can I make edits to a published email template in just one email that I am sending to one contact?

 

Using Legacy email I could have an email template set up that I could then make one-off changes via Contact > Email > Choose an Email > Edit > (make my edits) > Send Now.

 

In Advanced Mail after making edits I can only Publish and Send. But I don't want to make permanent changes to the template.

Seems like this feature (that I used all the time) has been lost in the new editor.

 

On top of that, any edits made in Advanced Mail don't actually appear in the sent email. They are saved to the template but not sent.

Anyone else having trouble with this? Support hasn't had other reports of these issues but has submitted a ticket.