Why it matters
If two pages don't both point to each other, Google ignores the tags; each variant must also list itself.
How to fix it
Generate the same alternates.languages map on every variant, including the page's own locale.
References
More seo rules
void seo reports 60 rules in this category. What search engines need from the raw response: status codes, titles and descriptions in <head>, self-referencing canonicals, one h1, crawlable links, Open Graph, icons, robots.txt and a sitemap with real dates.
link-textLinks with generic or empty textbroken-linksInternal links return 4xx/5xx (or go through redirects)trailing-slashBoth trailing-slash and non-slash URLs serve 200hreflang-invalidhreflang uses invalid codes or relative URLsog-missingRequired Open Graph tags missing (og:title, og:type, og:image, og:url)og-incompleteog:description / og:site_name missing, or og:url ≠ canonicalog-image-invalidog:image isn't a reachable, absolute, raster image within size limitsog-image-sizeog:image is smaller than 1200×630 or far from 1.91:1