Thanks for the detailed explanation. This helps. I think this is just my misunderstanding of how “hidden” communities work. I always thought that when a community is hidden, its posts just won’t show up in the Local/All/etc timelines. I wasn’t aware that community posts are hidden when you navigate to the community directly.

I use the Lemmy RSS feeds from openrss.org, which are different (and a little more robust) than the RSS feeds offered on the Lemmy instance. Some of the rss content has embedded links that go directly to a Lemmy community URL. So if I’m not subscribed to a hidden community and navigate to it, it’ll just show an empty page of posts, similar to your screenshot above.

So appears that this isn’t a problem with the RSS reader, but this behavior of a hidden community can surprise a person when they navigate to one and see no posts.