I think this is true for both platforms - instances don't federate communities (start polling for updates) until one user searches for that community. It takes a second search (at least in v17 lemmy, might be fixed in v18) to get the community to start showing. In Lemmy I think when an instance starts federating a community it also pulls the latest 20 posts, but given your example I'm guessing that must not be true for lemmy pulling in kbin magazines (not sure which side isn't doing it, I'd assume the kbin side since lemmy instances seem to be coming in with those 20 posts).
Fediverse
This magazine is dedicated to discussions on the federated social networking ecosystem, which includes decentralized and open-source social media platforms. Whether you are a user, developer, or simply interested in the concept of decentralized social media, this is the place for you. Here you can share your knowledge, ask questions, and engage in discussions on topics such as the benefits and challenges of decentralized social media, new and existing federated platforms, and more. From the latest developments and trends to ethical considerations and the future of federated social media, this category covers a wide range of topics related to the Fediverse.
Not sure if related but I've see this exact issue with my personal lemmy instance. Federation seems to be fine but to and from kbin.social seems half-broken. I can open the magazine and some have a single post but most are empty.
I also noticed that any moderation doesn't transfer between instances. I moderate @BestOf. I did remove one post for being irrelevant with the spirit of the community and it's still there on Lemmy. I hope this is something that can be fixed.
I've noticed some similarly weird synchronization issues when posting on [email protected]: some of my posts show up on the kbin side, but not on the canonical instance.
(Also, yay for m/dance! Am creator and would love some other contributors!)