If there are no more replies, then it just returns an empty string. For this post Radware Captcha Page what I’m seeing is 3 replies on the forum and 3 replies displayed on WordPress, so more_replies is returning an empty string. Are there cases on your site where there are more replies on the forum than are displayed on WordPress and the more_replies text is empty?
I see that I should add a filter to the more_replies text. I can do that right away.