vue-meta/docs/guide/caveats.md

Caveats

Reactive variables in template functions

Both title as meta support using template function. Due to how Vue.js determines reactivity it is not possible to use reactive variables directly in template function

{
  // this wont work
  metaInfo() {
    return {
      titleTemplate: chunk => (
        this.locale === 'nl-NL'
        ? `${chunk} - Welkom`
        : `${chunk} - Welcome`
      )
    }
  }
}

You need to assign the reactive variable to a local variable for this to work:

{
  // this will work
  metaInfo() {
    const locale = this.locale
    return {
      titleTemplate: chunk => (
        locale === 'nl-NL'
        ? `${chunk} - Welkom`
        : `${chunk} - Welcome`
      )
    }
  }
}