But when you first subscribe, you’d expect to be missing old posts
OP didn't expect to be missing old posts, hence his question. I had the same surprising discovery. Not sure how the UX could be improved to convey to the user what is actually happening.
Somewhat related, but why are we federating votes? Why not just federate the upvote count and downvote count? Does each server need to track the identity of every voter on a subscribed community?
Each server will track votes from their own users, preventing duplicate votes.
There is no easy way to do this. Each Lemmy instance uses their own autoincrement ID for posts. So post 12345 here might be post 54321 on another instance.
It has been suggested that each post gets a universal ID (UUID is an example). Then the local server could just examine the URL and redirect the user to the local post. This suggestion hasn't gotten any traction. The dev team is more focused on fixing the huge performance issues right now.