Servers not having the same content in their “all” feeds is not a bug, it’s by design. The design philosophy for Mastodon (and I’d say the fediverse as a whole) is to let the users curate their own feeds instead of showing them everything or algorithmically guessing what they might be interested in. Servers will only receive posts from accounts that at least one of this server’s accounts is subscribed to. Having every post federate to every server even if nobody there is interested in those posts would be a waste of resources.
Yes, that makes discovery of new content significantly harder but that’s the tradeoff for being able to host your own small instance without the need for a super powerful server. I can run my instance that serves just a couple of users on a 10-year-old server that runs a dozen other things at the same time. We see the stuff we’re interested in and don’t have to spend disk space, processing power and network bandwidth on content none of us will ever read and neither do we have to spend those resources on sending our posts to other instances where nobody will read them.
It’s “all” in the sense that it’s everything the instance knows about, in contrast to “this server” which is only content from users that are registered on this specific instance. Same concept as the “all” and “local” feeds on lemmy. I agree that a better name might reduce the confusion but I can’t think of a good one.