Mutator breaks factory?

I have a client model with a mutator:

    public function setCompanyNameAttribute($value)
    {
        return strtoupper($value);
    }

And a factory

$factory->define(Client::class, function (Faker $faker) {
   return [
        'company_name' => $faker->company,
        'first_name' => $faker->firstName,
        'last_name' => $faker->lastName,
        'primary_phone` => $faker->phoneNumber,
    ];
});

When the factory is called, I do not receive the company_name as an attribute:

            $client = factory(Client::class)->make();
dd($client);

----- 
#attributes: array:3 [
    "first_name" => "Anna"
    "last_name" => "Mills"
    "primary_phone" => "334-717-4998"
  ]

When I remove the mutator, all 4 attributes are present:

  #attributes: array:4 [
    "company_name" => "Weissnat, Hilpert and Osinski"
    "first_name" => "Jaylon"
    "last_name" => "Quitzon"
    "primary_phone" => "602-948-8484"
  ]

I added protected $fillable = ['company_name'] and have no $guarded but the company_name still doesn't show in the attributes list.

Is this a known issue, or do I need to add something else to my client model? Most of my tests broke with the addition of the mutator.

(edit: markdown...)