definitely a good place to ask this!

My linited understandign based on Lemmys docs (https://join-lemmy.org/docs/en/administration/federation_getting_started.html) is that only the community you subscribe to will federate:

If you search for a community first time, 20 posts are fetched initially. Only if a least one user on your instance subscribes to the remote community, will the community send updates to your instance. Updates include:

• New posts, comments
• Votes
• ...

That should explain the missing content as well: Lemmy only fetches a portion of a community at the time of subscribing, and only the content created after the initial federation will be sent to us - you can manually force a refresh by getting the link to a comment on the remote instance, and searching that on our instance - that'll get all the data about its parents, but that's admittedly pretty inconvenient. The process is described in more detail in the docs linked above!