I’ve only just noticed the same issue as everyone else, I DO have S2member installed, but that wasn’t the issue.
Turns out it was down to the Custom Search plugin by “bestwebsoft” https://wordpress.org/plugins/custom-search-plugin/
I’ve raised the issue on the WP support forum https://wordpress.org/support/topic/wp-40-breaking-bbpress-not-showing-1st-forum-post?replies=1#post-6172365
