It's how federation works, if you are the first from your instance you don't see posts, after some minutes the 2 servers communicate behind the scenes, and for your second visit everything looks normal.
When I first navigated to the community it said 0 posts, but when I refreshed they showed up. It still said 0, but I could see them. Leaving and coming back fixed it though. I don't know if that's on Lemmy's side or the app I'm using (Boost).