Metadata: parent title.template is applied to a page-level string title (Next treats it as absolute) #3065

What happened

vinext applies a parent layout's title.template to a page that exports a plain string title. Next.js 16 treats a page-level string title as absolute and does not apply the parent template, so the site name ends up duplicated under vinext.

Next.js 16.3.2   Home | Devfive
vinext           Home | Devfive | Devfive

Next.js 16.3.2   홈 | 데브파이브
vinext           홈 | 데브파이브 | 데브파이브

Only the two root routes (/en/, /ko/) differ; other routes match, because only these set a page-level string title.

Reproduction

app/[lang]/layout.tsx

export async function generateMetadata(): Promise<Metadata> {
  return {
    title: {
      default: 'Devfive',
      template: '%s | Devfive',
    },
  }
}

app/[lang]/page.tsx

export async function generateMetadata(): Promise<Metadata> {
  return { title: 'Home | Devfive' }
}

• Next.js renders <title>Home | Devfive</title> — the string is absolute, template skipped.
• vinext renders <title>Home | Devfive | Devfive</title> — template applied on top.

Our application deliberately writes the full title at page level precisely because Next 16 does not apply the template there (we have a code comment saying so, added while fixing an accessibility requirement for unique page titles). Under vinext that workaround double-applies.

Expected

Match Next.js: a page-level title: string is absolute; the parent title.template applies only to title.default resolution for child segments that do not set their own title, and to title: { absolute: ... } / nested title objects per the Metadata API rules.

Why it matters

<title> is user-visible in the tab and in search results, so this is an SEO/UX regression rather than a cosmetic difference. It was caught by a golden-master diff comparing SSR HTML between the two runtimes.

Environment

• vinext 1.0.0-beta.8, Vite 8.2.2
• Next.js 16.3.2 API surface, App Router, generateMetadata
• next-intl for localization (the title strings come from a plain record, not from next-intl, so the plugin is not involved)
• --platform=node, output: 'standalone'