[@piksel Participant 12 years, 9 months ago]

After a little digging, discovered my Custom Post Type Permalinks plugin wasn’t playing nicely with bbPress.

Problem and solution (albeit temporary) documented here: https://wordpress.org/support/topic/bbpress-topic-links-in-forums-break

Ultimately, edited the CPTP plugin file directly (ugh) line 290-something

$parentsDirs = "";
if( !$leavename ){
	$postId = $post->ID;
	if ( !in_array($post_type, array('topic', 'forum', 'reply') ) ) {
		while ($parent = get_post($postId)->post_parent) {
			$parentsDirs = get_post($parent)->post_name."/".$parentsDirs;
			$postId = $parent;
		}
	}
}