Overnight, since 16 September 2017, the Sharing feature in WordPress.com blogs appears to have become broken.
Previously, tweets generated automatically by WordPress.com and posted to Twitter contained the title of the article, its link, and the associated image (if there is one).
Currently, those automatic tweets contain only the link to the article: the title and image do not appear. The only way that I can see of restoring normal tweets is to post them manually.
Hopefully normal sharing service will be resumed shortly.
Postscript (0740 18 September 2017): the folk at WordPress.com have now fixed this problem, and automatic posts to Twitter should now be working normally again. Thank you, @wordpressdotcom for your help.