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I think there are two things that might be happening:

1. You are overwriting each year list on every run.
2. The properties of the item might not exist.

let titles = {}

this.user.titles.forEach(title => {
    let key = title.year_id

    const item = {
      id: title.id
      en: title.en
    }

    if(!titles[key]) {
      titles[key] = []
    }

    titles[key].push(item)
})

return titles

If you want to copy all props for a title you can use this instead:

const item = { ...title }