bbPress.org

I went to clan.wolflan.com and your source code looks like some paragraphs were added by your editor as I see some <p> and </p> in weird places. What software did you use to edit loop-single-forum.php or your functions.php ?

I was not able to check because some of your forums are private, but what’s the html source code for the list of subforums ? (I mean in the browser) Is there a <br /> between the list of subforums ? Because, when I watch your screenshot, subforums are not separated by a comma, so I guess the function did his job, but your code may be broken because you used WordPress editor or an other editor that deleted the <br />.

Anyway, in your case, the simplest solution may be using some CSS. And as you said, simple stuff first ! You wouldn’t need to change any php template. You may try something like that in your style.css or Custom CSS if you use Jetpack :

#bbpress-forums .bbp-forums-list li {
  display: block;
}

If it works, I don’t know what was the problem with the bbp_list_forums() function, but you will not need to use it and you can delete the custom loop-single-forum.php that you copied to your theme’s directory if you had not made other edits.