bbPress.org

@omj-1

This is the CSS causing the issue.

#bbpress-forums .bbp-body div.bbp-reply-content {
    clear: both;
    margin: 10px;
    padding: 0;
}

The reason why when reply threading is enabled fixes the issue is because the list .bbp-body self closes and makes the above CSS not work.

Here is the ticket about the reply threading markup bug.

https://bbpress.trac.wordpress.org/ticket/2757

You can use this custom CSS to fix the issue.

#bbpress-forums .bbp-body div.bbp-reply-content {
    clear: none;
}