I am not an admin or a long time lemmy user. I have been here for like 3 weeks. If other people have better answers they are likely correct. Your post in this thread was asking why the communities don't match and you didn't mention anything about them being defederated. The point of my comment was to point out that that was likely why and give a quick summary of what I understood about it. I was just regurgitating what I understood from my reading on this issue. I am/was very possibly not 100% accurate, I was just hoping to point you in the right direction.
I am both commenting on this post and referencing the one you linked in your reply. But I think this is the meat and potatoes from the admins post that explains how it works. I also think by reading some other comments in that thread that the admins aren't 100% correct maybe(?) Because it seems that we may be in a read only state, like, we still receive their content/comments, they just don't get ours. There is a paragraph I bolded that somewhat explains the phenomenon you are talking about.
Another one I bolded explains you can still see posts on beehaw communities, but they are from lemmy.world users? Maybe double check you aren't seeing lemmy.world users posts in those communities?
The gist of it is when you look at a post/community you are looking at a version hosted on your instance. We (lemmy.world) are scrubbed from existence on the beehaw.org hosted version of posts, but they aren't scrubbed from our version.
How defederation works
Now take that example post from earlier, the one on beehaw.org. The “true” version of the post is on beehaw.org but the post is still hosted on both instances (again, it has a copy hosted on all instances). Let’s say someone with an account on beehaw.org comments on that post. That comment is going to be sent to every version of that post via ActivityPub, as the “true” version has been updated. That is, every version EXCEPT lemmy.world and sh.itjust.works. So users on lemmy.world and sh.itjust.works won’t get that comment, because we’ve been defederated from beehaw.org. If we write a comment, it will only be visible from accounts on lemmy.world, because we posted to a copy, but our copy is now out of sync with the “true” version. So we can appear to interact with the post, but those interactions are ONLY visible by other lemmy.world accounts, since our comments aren’t send to other versions. As the “true” version is hosted on beehaw, and we no longer get beehaw updates due to defederation, we will not see comments from ANY other community on those posts (including from other defederated instances like sh.itjust.works).
The same goes for posting to beehaw communities. We can still do that. However, the “true” version of those communities are the ones on beehaw, so our posts will not be shared to other instances via ActivityPub. And all of this is true for Beehaw users with our communities. Beehaw users can continue to see and interact with Lemmy.world communities, but those interactions are only visible to other Beehaw users, since the “true” versions of the Lemmy.world communities (the ones sent to/synced with every other instance) is the Lemmy.world one.
Communities on other instances, for example lemmy.ml, are unaffected by this. Lemmy.world and beehaw.org users will still be able to interact with those communities, but posts/comments from lemmy.world users won’t be visible to beehaw.org users, as defederation prevents our posts/comments from being sent to the version of these posts hosted on beehaw.org. However, as the “true” version is the one on the third instance, we can still see everything from beehaw.org users. So we see a more filled in version than the beehaw users.
Why can I still see posts/comments from beehaw users?
Until they defederated us, posts/comments were being sent to lemmy.world, so we can see everything from before defederation. After defederation, we are no longer receiving or sending updates. So there are now multiple versions of those posts.
Why can I still interact with beehaw communities?
This won’t ever stop. You’ll notice that all posts after defederation are only from lemmy.world users. You won’t see posts/comments from ANY other instance (including instances that ) on beehaw.org communities.
Those communities will quickly suck for us, as we’re only talking to other lemmy.world users. Your posts/comments are not being sent to any other lemmy. I highly recommend just unsubscribing from those communities, since they’re pretty pointless for us to be in right now.
Why do I still see comments from beehaw users on lemmy.world communities?
Again, comments from before defederation were still sent to us. After defederation, it will no longer be possible for beehaw users to interact with the “true” version of lemmy.world communities. Their posts/comments are not being sent to any other lemmy. They also aren’t getting updates from any other lemmy, as the “true” version of those communities is on our instance.
Why do I see posts/comments from beehaw users on communities outside lemmy.world and beehaw.org?
That’s because the “true” version of those posts is outside beehaw. So we get updates from those posts. And lemmy.world didn’t defederate beehaw, so posts/comments from beehaw users can still come to versions hosted on lemmy.world.
The reverse is not true. Because beehaw defederate lemmy.world, any post/comment from a lemmy.world users will NOT be sent to the beehaw version of the post