@torparskytt yep. That's how mastodon works. It can't show you replies from accounts it doesn't know. You loaded the post from the other users instance through Mona. Therefore, the users that your instance didn't know about, loaded on their instance.
@BeAware Seems like it would be very doable for an app developer to automatically load replies from the original post’s instance whenever you open/look at a post, but I’m no developer.