I contacted the XenForo team and identified a bug I discovered in their latest patch. The bugs behavior is like this; when pasting a URL in a thread/post, it automatically creates a hyperlink. Because of this, the UnFURL does not work. While we wait for a fix, all you have to do is right-click and select "paste as plain text" and this will prevent a hyperlink being created. The UnFURL should display properly once you save the post. I will advise once the bug has been fixed.