Take a look here: https://bbpress.org/forums/topic/roles-and-capabilities-in-bbpress-2-2/
I fixed my disappeared forum using the forums role repair tool, located in: Admin > Tools > Forums
I have the same issue of Annie S Spratt.
My “Private” forum is gone, the forum index (www.my-domain.it/forums/forum/name-of-the-forum/) tells me “Sorry, no posts matched your criteria.”
But topics are reachable via a direct link (making the forum public, taking the url of a topic and then making again the forum private)