BuddyPress.org

We may need to be clearer on the steps to replicate this as…
With a twentyseventeen theme, BP trunk, stable bbpress no other plugins.

With an index page displaying the forums list shortcode & search input I can search for forum topics quite happily, returning results to the page showing bbp results.

However tested on single site not MS and use of directory MS rather than sub-domain might be a factor.