I made a post to funny@Lemmy.world, then in another tab I subscribed to science@lemmy.ml. Back in the tab I made the post in, the page updated itself to look like I made the post in science@lemmy.ml instead. I didn’t refresh the page or do anything else… It just changed by itself.
More testing: Seems to happen when I sub to communities on other instances, not my home server Lemmy.world.