@ololduck searching hashtags will only work for content that your instance would otherwise have seen, so if you're on a single user instance, you'll only see results for hashtags for posts that were already in your feed as that is the totality of the posts your instance knows about.

OTOH when you post with a hashtag, that post is visible to people searching that tag on all instances where at least one of their users already follows you, as your posts are federated to their instance already.