Interestingly, this problem appears when accessing topics from a category. When accessing topics from the homepage of the site (categories index, latest posts, etc.), the topic page appears to apply the body class accordingly.
I assume it has to do with the way the web application is rendering the topic pages.
The problem can be seen my visiting this category and clicking on to a topic.
http://www.helloforos.com/c/entretenimiento/chismesgordos