Template suggestion used from module folder

Hello,

i have the following hook_theme()

function mymodule_theme() {
  return [
    'event' => [
      'render element' => 'elements',
    ]
  ];
}

I also have the following hook_theme_suggestions_HOOK()

function mymodule_theme_suggestions_event(array $variables) {
  $event = $variables['elements']['#event'];
  $sanitizedViewMode = $variables['elements']['#view_mode'];

  $suggestions = [];
  $suggestions[] = 'event__' . $sanitizedViewMode;
  $suggestions[] = 'event__' . $event->id();
  $suggestions[] = 'event__' . $sanitizedViewMode . '__' . $event->id();

  return $suggestions;
}

My Controller-Function:

public function showEvent(Event $event, $view_mode = 'small') {
    $build = ['#theme' => 'event'];
    $build += \Drupal::entityTypeManager()->getViewBuilder('event')->view($event, $view_mode);

    return $build;
}

When I now create a file modules/custom/mymodule/templates/event--small.html.twig it isn't used for rendering.
If I move the file "event--small.html.twig" to the theme-folder (e.g. core/themes/bartik/templates/event--small.html.twig), everything works well.
How can I achieve, that the files from the modules folder are used?

br
Daniel Steindl