Managed file element in custom field module doesn't save target id

I have been breaking my head on this issue for a few days now. Using Drupal 9.2.x, I am building a custom field module for a client that has multiple fields. I isolated the problem by only including a managed file field for now, because the other fields are not causing this issue.

I defined a managed file element in my Widget class, which extends WidgetBase:

public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {

    $element['target_id'] = [
      '#type' => 'managed_file',
      '#title' => $this->t('Right image'),
      '#upload_location' => 'public://disco-field-images/',
      '#upload_validators' => array(
        'file_validate_extensions' => array('gif png jpg jpeg')
      ),
      '#default_value' => array($items[$delta]->target_id),
    ];

    return $element;

  }

I defined my schema as follows in my Item class:

 public static function schema(FieldStorageDefinitionInterface $field_definition) {
return [
  'columns' => [
    'target_id' => [
      'description' => 'The ID of the file entity',
      'type' => 'int',
      'unsigned' => TRUE,
    ],
  ],
  'indexes' => [
    'target_id' => ['target_id'],
  ],
  'foreign keys' => [
    'target_id' => [
      'table' => 'file_managed',
      'columns' => ['target_id' => 'fid'],
    ],
  ],
];

}

And finally, I have added a property defintion for the field:

public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {

$properties['target_id'] = ListDataDefinition::create('integer')
  ->setLabel(t('target ID'))
  ->setRequired(FALSE);

 return $properties;

  }

Somehow this results in the file being uploaded into the managed files table, getting a fid and all looks well from there. However, the field table (node__fieldname) which contains the referencing target_id to the file_managed table somehow always defaults to 1. So no reference is being made to the uploaded file. I have tried working with other property definitions:

    $properties['target_id'] = DataDefinition::create('integer')
  ->setLabel(t('target ID'));

and

$properties['target_id'] = DataReferenceTargetDefinition::create('integer')
 ->setLabel('Foreign key referencing managed file')
 ->setSetting('unsigned', TRUE)
 ->setRequired(FALSE);

But no luck there. It seems a bit weird to me though, that the database target id needs to be integer and the file upload widget works with an array. Thats why I am aiming at the property definitions/schema as a cause here. If I try to work with a single int property value:

DataDefinition::create('integer')

I get a primitive type error because the managed file element expects an array. Any suggestions? Thanks!