Hi aliwari,
This variable does not show the module class when you add from the backend > mod_articles_category module, if you want to resolve this issue, you can add the following code to the wwwroot\templates\ja_seven\html\mod_articles_category\list.php file, line 15:
$moduleclass_sfx = $params->get('moduleclass_sfx') != null
? htmlspecialchars($params->get('moduleclass_sfx')) : '';
Or wait for our technical team to update this issue in the next version.