I just posted this to Hubski, and realized after posting I needed to edit a sentence for clarity.
Clicked the Edit link, and the URL for the tweet was the only thing in the Edit Text field... everything else was gone. Had to retype the rest of the post.
I suspect it may be because the URL for the Tweet was the first line of my post, with no other text or formatting. I have NOT tested whether the problem also exists if the Tweet is embedded deeper into the text. (I suspect it doesn't happen then; probably something about an embedded tweet being the first line of the content section of the post, I expect.)