if you can give a specific example I can look at it and bug the admins with my findings but I’m guessing its just relatively low-activity comms that haven’t had any of their backlog synced over (lemmy only syncs like 20 posts per comm by default, and all new posts after that point should also sync)
federation is weird. lemmy doesn’t sync all posts when you federate, it only fetches communities when someone specifically searches for them and even then it only grabs the last 20 posts when it does that. i think there are bots to automate syncing comms/maybe post backlogs too, some maybe even usable as an end user
as in historical posts?
if they were visible from lemmy.sdf.org before that means they made it into the local database over there so they should only disappear if deleted