Reply comment is not deleted after the mian comment gets deleted

Hello, is this normal behavour? For example, I have two comments: one is a main comment and the other is a reply comment. When I detete the main comment, only the main one get deleted but not the reply one. In the frontend they both however disapear but in the backend only the main comment’s gone but the reply one’s still there. So, I have the code below to display a comment count in a content template but it displays the comment count as (1), with the absense of comment. This doesn’t make sense.

<?php echo get_comments_number($post_id);?>

How can I delete a main comment and all its reply comments get deleted too so that the comment count displays correctly as zero?