Unable to add more than 128 characters worth of dates

I'm using this hook in my module:

/**
 * Implements hook_field_widget_single_element_WIDGET_TYPE_form_alter().
 */
function MYMODULE_field_widget_single_element_multiple_dates_default_widget_form_alter(array &$element, FormStateInterface $form_state, array $context) {
  $element['multiple_dates']['#maxlength'] = 255;
}