delayedinsanity, you beauty. As far as I can see, this is the problem. I switched to using wp-load.php and the status in the header is 202 without my hack. What’s more, it corrects this issue:
https://bbpress.org/forums/topic/wpmu-284a-deep-integration-breaks-rss-feed
Many thanks.
So to sum it up. I was previously doing the following in bb-config.php for deep integration:
require_once('/var/www/vhosts/youlookfab.com/httpdocs/wp-blog-header.php');
But it broke RSS feeds and put 404 headers in my forum pages. The following seems to solve both issues:
require_once('/var/www/vhosts/youlookfab.com/httpdocs/wp-load.php');
