That's because SSR is enabled. And photon doesn't have access to the Lemmy database completely, so it can't figure out what account you're using until the client fully loads Photon. So what's happening is this:

Server: loads posts with default settings

client starts loading...

Client: takes over server and loads posts client side using the users settings.

This will lead to different posts depending on what sort you use, what location, and if it differs from the server.