This has always been a flaky thing. Federation has worked in the past but not reliably. No idea why.
Looking into it now, it still seems incomplete. I compared bookstodon with my view on mastodon (hachyderm.io) and the view that kbin.social has.
All three were different. Yay decentralisation and federation!!
Mastodon has the most content, which makes sense, a.gup.pe is intended to work with mastodon and so they'd make sure that works. Both lemmy and kbin seem to be missing content, which also makes sense as they're much smaller than mastodon and don't get the same attention from a.gup.pe (Yay ... federation).
Kbin.social seems to have more content than lemmy. But it also renders it as a microblog ... which is not how I want groups to be structured (hence why I'm on the threadiverse/lemmy/kbin).
Lemmy's a.gup.pe feed being incomplete is likely a fundamental issue with how a.gup.pe structure their posts and lemmy not understanding that. I'm guessing Lemmy's rule of disallowing posting to multiple targets/communities is a big factor in this, as a.gup.pe posts are typically made by mastodon users who are likely to also tag other users, whereas lemmy, to prevent brigading, will only accept a community/group as the first address in a post (yay .... federation ....).
So while it's good seeing this liven up again ... it seems like someone needs to get into the weeds and sort this out from both sides.
Otherwise, and maybe I'm feeling particularly grumpy about the fediverse today ... this really is a great demonstration of how decentralised/federated social media can be ... just meh and a bit shit. It shouldn't be this hard to implement a standard structure for social media (groups/channels/sub-reddits) with an allegedly standardised protocol. In reality, it seems, the standard is rather ambiguous and flimsy to the point that every platform is idiosyncratic.
I say this just so that any one getting excited about possibilities doesn't get burnt out from incomprehensibly annoying disappointments. They are all around the place and can darken this place a bit.