Update – using permalinks fixes things for me as well, except the URI is https://www.blogname.com/feed/ which doesn’t help all the people who were using /wp-rss2.php and are getting a 404 error. Going back to no permalinks doesn’t fix it either.
But of course. Other than Carthik’s Update Linkroll plugin (now deactivated with no joy), I haven’t changed anything but the index.php page. Everything else is fresh and new. I’m going to try a brand new install on another server to see if it duplicates.