I read elsewhere that it’s not completely bidirectional when it comes to posts on a third party instance. For example, if lemmy.world and beehaw.org users both post to a thread on another instance that they are both still federated with, lemmy.world users will see the beehaw.org users’ posts, but not the other way around.

I haven’t confirmed if this is true yet though, so maybe someone can correct me if I’m wrong.