A Hyvä component shows no price or stock, is that a GraphQL bug?

Usually, yes. Hyvä components fetch price, stock and cart data over GraphQL, so a missing value almost always means the GraphQL response carries an error or returns null, GraphQL response contains errors: Cannot query field. Common causes: a custom resolver throwing, a schema field renamed after an upgrade, a missing @cache / @resolver directive, or full-page cache serving stale GraphQL. We inspect the network tab, replay the query, and fix the resolver or schema so the data renders again.