dd($items['acf']['items']);
  //produces array:2 [▼
  0 => array:2 [▶]
  1 => array:2 [▶]
]

collect($items['acf']['items'])->each(function($chapter) use ($chapters){
                   $chapters->push((object) array('title' => $chapter['title'], 'timestamp' => $chapter['timestamp']));
                });

Undefined index: items