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.
If it matters: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0) Gecko/20100101 Firefox/114.0
More testing: Seems to happen when I sub to communities on other instances, not my home server Lemmy.world.