A couple of years ago, I moved my word press blog down a directory level, from domain.com to domain.com/blog. I then did a redirect so that any path to an old page ended up going down to the right path. For example, an old blog post like domain.com/2006/10/01/post resolved to domain.com/blog/2006/10/01/post. And it was done so that I can add new website pages, and they did not trigger the redirect.
I recently upgraded my wordpress software and theme, and now the redirect isn't working. All my old blog posts just go to domain.com/blog. For example, domain.com/2006/10/01/post goes to domain.com/blog, not to domain.com/blog/2006/10/01/post. For all I knew it's been happening for a while.
I checked my .htaccess, and there is nothing there I can find that would redirect the way I need, and frankly I don't remember what I did to get it working before. Perhaps it was some interaction with 404. Perhaps it was an interaction with the database backend, which was upgraded automatically. I don't know.