The app doesn't decide which community to post in; you do. When you pick a community for the post to go to, you might pick [email protected], in which case it becomes hosted by the beehaw.org server. Or you might pick [email protected], in which case it's hosted by the lemmy.world server. If you vote or comment on the post that's on beehaw.org, the vote/comment is hosted there as well, and likewise for lemmy.world.
Yes! It's a bug. The post suddenly changes, while I'm writing a comment. I also deleted it immediately, but that didn't seem to federate properly. I resurrected and edited it, hoping that'll update for everyone. Original intended context: https://lemmy.world/post/129333