Data which has been appended to the current Scrivener document via the provided service does not get registered in the Undo queue.
This was expected behaviour - how I had programmed it. However, I have now changed it for beta 2 so that it is undoable.
Please don’t poke around with undo too much in Scrivener. 
You will find that the undo stack often gets cleared when dealing with stuff in the binder. This is because Scrivener actually deals with creating and deleting files off disk, so I choose to clear undo in situations where an undo should otherwise lead to a file being deleted from disk. That said, each piece of text (“scrivening”, if you will ) has its own undo manager, so that even if the undo stack gets cleared in the binder, whenever you return to a document that you have opened in that session, you will be able to continue to undo your edits - a little like how in Photoshop each image you have open has its own undo stack.
Thanks,
Keith