@petunia each Fediverse instance only shows hashtag results for posts it has actually seen. A newly created or small instance (like theforkiverse) simply hasn’t federated with as many accounts and servers yet, so it has a much smaller set of posts in its local index of that tag.
If you want to see all the posts on a hashtag across the Fediverse, the only reliable way right now is to check a larger or more connected instance, because no instance has a truly complete global view of every hashtag.