PHP - Laravel returning string instead of array (JSON)

I have a model called CroppedDocumentField, that have below $casts setup:

protected $casts = [
    'content' => 'array'
];

The migration looks like this:

Schema::create('cropped_document_fields', function(Blueprint $table){
      $table->increments('id');
      $table->unsignedInteger('document_id');
      $table->json('content')->nullable();
});

In my database, the content column seems to be stored like a string:

"{\"1\": [{\"row\": \"Bill Tc\\n\"}, {\"row\": \"Nathar\\n\"}, {\"row\": \"75839\\n\"}]}"

If I echo that out:

$document = CroppedDocumentField::Find(56);
dd(is_array($document->content));

This returns false.

When I insert the JSON to my database, I read it from a .txt file, that contains the JSON string:

{"1": [{"row": "Bill Tc\n"}, {"row": "Nathar\n"}, {"row": "75839\n"}]}

Then I insert it:

$file = "mytext.txt";
$content = file_get_contents($file);

//Add the text content
$this->document->cropped()->create([
     'content' => $content
]);

In my document model, I simply have a relationship to the CroppedDocumentField model:

//Document.php:
public function cropped()
{
    return $this->hasMany(CroppedDocumentField::class);
}

What am I doing wrong here?