Looks like you have a directory titled “forum” within your forum directory.
That is standard core behavior right now. You would have to hack the core files or come up with a plugin to go around it.
I couldn’t find where is it described as expected behaviour. This does seem pretty weird?
It would be more accurate if it read “forums/sub-forum/topic” but it really is just a minor cosmetic issue.
It’s actually a pretty simple hack, though I caution you that I worked it out in the last five minutes.
First, modify the following three lines of your bbpress .htaccess;
RewriteRule ^forum/([^/]+)/page/([0-9]+)/?$ /bbpress/forum.php?id=$1&page=$2 [L,QSA]
RewriteRule ^forum/([^/]+)/?$ /bbpress/forum.php?id=$1 [L,QSA]
RewriteRule ^forum/?$ /bbpress/ [R=302,L,QSA]
change ^forum/ to whatever you want the permalink to be, such as ^sub/ or ^purplepolkadots/, whatever works for you. Then open functions.bb-template.php and go to line 711 inside function get_forum_link() (line numbers may vary). It should look something like:
$link = bb_get_uri('forum/' . $forum->$column . $page, null, $context);
…and change ‘forum/’ to whatever you put in your .htaccess – this seemed to work for me immediately, however as I mentioned, I haven’t done any thorough testing of it.
Sam told me this is a throwback to some of the older versions of the software. Hopefully it will be fixed in a future version of bbPress.
