Relationships are lost when converting to array

I'd a model with following relation:

customField [1:n] customFieldType [1:n] customFieldTypeListValues

I load the relations:

        $customField = \App\Models\CustomField::where('id', 1)->first();
        $customField->load('customFieldType.customFieldTypeListValues');

When I load a record with relations, dd shows me all relations:

App\Models\CustomField {#1706 ▼ // app/Http/Controllers/Document/CustomFieldController.php:71
  #connection: "mysql"
  #table: "custom_fields"
  #primaryKey: "id"
  #keyType: "int"
  +incrementing: true
  #with: []
  #withCount: []
  +preventsLazyLoading: false
  #perPage: 15
  +exists: true
  +wasRecentlyCreated: false
  #escapeWhenCastingToString: false
  #attributes: array:7 [▼
    "id" => 2
    "custom_fieldable_type" => "App\Models\Document"
    "custom_fieldable_id" => 1
    "custom_field_type_id" => 2
    "value_string" => ""
    "created_at" => "2024-09-16 13:19:02"
    "updated_at" => "2024-09-16 14:03:26"
  ]
  #original: array:7 [▶]
  #changes: []
  #casts: array:3 [▶]
  #classCastCache: []
  #attributeCastCache: []
  #dateFormat: null
  #appends: array:1 [▶]
  #dispatchesEvents: []
  #observables: []
  #relations: array:1 [▼
    "customFieldType" => App\Models\CustomFieldType {#2302 ▼
      #connection: "mysql"
      #table: "custom_field_types"
      #primaryKey: "id"
      #keyType: "int"
      +incrementing: true
      #with: []
      #withCount: []
      +preventsLazyLoading: false
      #perPage: 15
      +exists: true
      +wasRecentlyCreated: false
      #escapeWhenCastingToString: false
      #attributes: array:7 [▶]
      #original: array:7 [▶]
      #changes: []
      #casts: array:5 [▶]
      #classCastCache: []
      #attributeCastCache: []
      #dateFormat: null
      #appends: []
      #dispatchesEvents: array:2 [▶]
      #observables: []
      #relations: array:1 [▼
        "customFieldTypeListValues" => Illuminate\Database\Eloquent\Collection {#2308 ▼
          #items: array:1 [▼
            0 => App\Models\CustomFieldTypeListValue {#2309 ▼
              #connection: "mysql"
              #table: "custom_field_type_list_values"
              #primaryKey: "id"
              #keyType: "int"
              +incrementing: true
              #with: []
              #withCount: []
              +preventsLazyLoading: false
              #perPage: 15
              +exists: true
              +wasRecentlyCreated: false
              #escapeWhenCastingToString: false
              #attributes: array:6 [▼
                "id" => 1
                "custom_field_type_id" => 2
                "value" => "Wert 1"
                "created_at" => "2024-09-16 13:19:09"
                "updated_at" => "2024-09-16 13:19:09"
                "archived_at" => null
              ]
              #original: array:6 [▶]
              #changes: []
              #casts: array:3 [▶]
              #classCastCache: []
              #attributeCastCache: []
              #dateFormat: null
              #appends: []
              #dispatchesEvents: []
              #observables: []
              #relations: []
              #touches: []
              +timestamps: true
              +usesUniqueIds: false
              #hidden: []
              #visible: []
              #fillable: array:2 [▶]
              #guarded: array:1 [▶]
            }
          ]
          #escapeWhenCastingToString: false
        }
      ]
      #touches: []
      +timestamps: true
      +usesUniqueIds: false
      #hidden: []
      #visible: []
      #fillable: array:3 [▶]
      #guarded: array:1 [▶]
    }
  ]
  #touches: []
  +timestamps: true
  +usesUniqueIds: false
  #hidden: []
  #visible: []
  #fillable: array:5 [▶]
  #guarded: array:1 [▶]
}

When I'm using toArray(), I get following result:

array:9 [▼ // app/Http/Controllers/Document/CustomFieldController.php:71
  "id" => 2
  "custom_fieldable_type" => "App\Models\Document"
  "custom_fieldable_id" => 1
  "custom_field_type_id" => 2
  "value_string" => ""
  "created_at" => "16.09.2024 13:19:02"
  "updated_at" => "16.09.2024 14:03:26"
  "value" => array:1 [▶]
  "custom_field_type" => array:7 [▼
    "id" => 2
    "name" => "liste"
    "data_type" => "list_single"
    "is_required" => false
    "created_at" => "16.09.2024 13:18:58"
    "updated_at" => "16.09.2024 13:18:58"
    "archived_at" => null
  ]
]

As you can see, the relation custom_field_type_list_values is missing in the array. I inspect the framework code and all I see is that:

        return array_merge($this->attributesToArray(), $this->relationsToArray());

That irritated me because it should actually work. I rebuilt it by hand:

        $customField = \App\Models\CustomField::where('id', 1)->first();
        $customField->load('customFieldType.customFieldTypeListValues');
        $customFieldArray = array_merge($customField->attributesToArray(), $customField->relationsToArray());

Same result! Then I did this:

        $customField = \App\Models\CustomField::where('id', 1)->first();
        $customField->load('customFieldType.customFieldTypeListValues');
        $customFieldRelations = $customField->relationsToArray();
        $customFieldArray = array_merge($customField->attributesToArray(), $customFieldRelations);

And now the relations are loaded correctly:

array:9 [▼ // app/Http/Controllers/Document/CustomFieldController.php:73
  "id" => 2
  "custom_fieldable_type" => "App\Models\Document"
  "custom_fieldable_id" => 1
  "custom_field_type_id" => 2
  "value_string" => ""
  "created_at" => "16.09.2024 13:19:02"
  "updated_at" => "16.09.2024 14:03:26"
  "value" => array:1 [▶]
  "custom_field_type" => array:8 [▼
    "id" => 2
    "name" => "liste"
    "data_type" => "list_single"
    "is_required" => false
    "created_at" => "16.09.2024 13:18:58"
    "updated_at" => "16.09.2024 13:18:58"
    "archived_at" => null
    "custom_field_type_list_values" => array:1 [▼
      0 => array:6 [▼
        "id" => 1
        "custom_field_type_id" => 2
        "value" => "Wert 1"
        "created_at" => "16.09.2024 13:19:09"
        "updated_at" => "16.09.2024 13:19:09"
        "archived_at" => null
      ]
    ]
  ]
]

What's going wrong here? Am I making a mistake? Why is there a different behavior here when I call a method previous?