Debugging "UNCACHEABLE" header

We have a site where the homepage is not being cached and contains the headers:

x-cache: MISS, MISS
x-cache-hits: 0, 0
x-content-type-options: nosniff
x-drupal-dynamic-cache: UNCACHEABLE

I narrowed this down to the content regions contents, and disabled the "Main page content" for the front page. This then gave me a cache HIT, and no longer responded as UNCACHEABLE. From there, I narrowed it down to a field formatter being used on a paragraph. We have a custom one that extends the normal entity render formatter.

If I swap it back to the original "Render entity" formatter, everything is fine. So then, it must be something we are doing in this custom formatter causing the issue.

I can see when I follow with xdebug that shouldCacheResponse of DynamicPageCacheSubscriber returns FALSE, because something is setting max-age to 0 (not by code). It looks like calls to addCacheableDependency may be triggering this behavior in the formatter:

Essentially, the formatter adds cache data to the render so if any of its referenced items are updated, the cache should be invalidated for that host paragraph so it re-renders:

     $view_builder = \Drupal::entityTypeManager()->getViewBuilder($entity->getEntityTypeId());
      $elements[$delta] = $view_builder->view($entity, $view_mode, $entity->language()->getId());

      try {
        $parent = $items->getParent();
        $parent_entity = $parent->getValue();
        $elements[$delta]['#cache']['keys'][] = $parent_entity->id();
        $elements[$delta]['#cache']['keys'][] = $parent_entity->bundle();
        $elements[$delta]['#cache']['keys'][] = $parent_entity->getEntityTypeId();
        $elements[$delta]['#cache']['keys'][] = 'delta_' . $delta;
        $elements[$delta]['#cache']['keys'][] = 'context_aware';

        $this->renderer->addCacheableDependency($elements[$delta], $parent);

        if ($entity->hasField('field_author')) {
          $child = $entity->field_author->entity;

          if (isset($child)) {
            $this->renderer->addCacheableDependency($elements[$delta], $child);
          }
        }

        // similar statements with addCacheableDependency

If I comment out this initial line:

$this->renderer->addCacheableDependency($elements[$delta], $parent);

Then I get a cacheable response. This looks to be because the $parent item (even though it is a node or paragraph or media entity) triggers this:

  /**
   * Creates a CacheableMetadata object from a depended object.
   *
   * @param \Drupal\Core\Cache\CacheableDependencyInterface|mixed $object
   *   The object whose cacheability metadata to retrieve. If it implements
   *   CacheableDependencyInterface, its cacheability metadata will be used,
   *   otherwise, the passed in object must be assumed to be uncacheable, so
   *   max-age 0 is set.
   *
   * @return static
   */
  public static function createFromObject($object) {
    if ($object instanceof CacheableDependencyInterface) {
      $meta = new static();
      $meta->cacheContexts = $object->getCacheContexts();
      $meta->cacheTags = $object->getCacheTags();
      $meta->cacheMaxAge = $object->getCacheMaxAge();
      return $meta;
    }

    // Objects that don't implement CacheableDependencyInterface must be assumed
    // to be uncacheable, so set max-age 0.
    $meta = new static();
    $meta->cacheMaxAge = 0;
    return $meta;
  }

Setting cacheMaxAge to 0 because it is not an instance of CacheableDependencyInterface.

If I am already setting the cache keys, is this line even needed:

$this->renderer->addCacheableDependency($elements[$delta], $parent);

If I remove that, will there be an adverse effect (like render displays not re-rendering when referenced items are saved)?