Well, it wasn’t me, it was a Real Life User.
But that said, the Gmail UI makes this the suggested workflow, because the big “Click here to Reply or Forward” box is below the most recent message in the thread, which is your own, in this scenario. The reply still goes to the other participant in the conversation, which of course is discourse+key@example.com, but it’s replying “to” your previous message (if you expand the hidden/collapsed portion of the message).