You can add it back very easy, putting this hook in the WordPress active theme functions.php file or using other custom code inserting solutions:

add_action('wpdiscuz_comment_form_before', function() {
   echo '<h3 style="max-width: 750px;margin: 0px auto;">' . __('Leave a Reply', 'wpdiscuz') . '</h3>';
});

There is no any bug here. This is the new design of wpDiscuz Comment Form:

Excellent update, loving the look and feel of it!

I have one issue that I'm not sure is an issue or not: the header is gone. It used to say "Leave a Reply" that I changed to "Comments". This header is gone. Previously, you recommended we change the header in the form field. However, this form field now covers "Comment form header text (singular)" and "Comment form header text (plural)" which sadly only change the small header next to the amount of comments which is below the comment box. I do not see any header option and it just seems weird that there is not one.

I'm unsure if this is a bug or a design choice to remove the header. I have searched the form over extensively and do not see anywhere to remedy this nor in the settings.

Any advice? Or custom code so I can re-add it?

Thanks!