@edent The problem seems to be caused by the <content> element in the Atom feed being a complete html page including <html> and <body> tags. I am sure that will confuse some phplist processing that does simple string matching. I can now repeat the problem.
I looked at the RSS feed and that too has those extra elements so I guess that they are in the actual Wordpress article. Posts in another Wordpress site that I help with are not complete html pages. Can you change the way that the articles are created to avoid this?