i18n.config.ts not recognized automatically (missing fallback locale)

[Tienisto]

Environment

• Operating System: Darwin
• Node Version: v22.13.1
• Nuxt Version: 3.15.4
• CLI Version: 3.21.1
• Nitro Version: 2.10.4
• Package Manager: [email protected]
• Builder: -
• User Config: modules, devtools, app, site, i18n, nitro, compatibilityDate
• Runtime Modules: @nuxt/[email protected], @nuxtjs/[email protected], @nuxtjs/[email protected], @nuxtjs/[email protected]
• Build Modules: -

Reproduction

i18n: {
    // vueI18n: './i18n.config.ts', <-- adding this works again
  locales: [
      {
        code: "de",
        language: "de-DE",
        file: "de.json",
        name: "Deutsch",
      },
      {
        code: "en",
        language: "en-US",
        file: "en.json",
        name: "English",
        isCatchallLocale: true,
      },
  }
}

i18n config file i18n.config.ts:

export default defineI18nConfig(() => ({
    fallbackLocale: 'en',
}))

Describe the bug

No fallback is executed indicating that i18n.config.ts was not properly loaded.

Additional context

localsend/website@5ba397b
For example, Thai (th) language

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