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).

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 japaneselanguage@sopuli.xyz: 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!)

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.