I think if you search your small instance for a post URL it fetches the post content from the server that hosts the community. Or maybe you have to visit the post url on your server (which has a weird format like https://your.instance/@other.instance/post/id or something vaguely similar.

There's no mass backfill, you could maybe script something using this. But I'd advise to do it sparingly, like the last 100 posts for your favorite communities or something. This kind of thing could induce unreasonable load on the community server if lots of people did it, which is why Jerboa doesn't backfill by default.

Also note, this is a temporary problem. How often do you scroll back more than a week? By next week, your server will have that much history.