Q:

Exclude posts from search results based on a meta_query with "NOT EXISTS" doesn't work anymore

I have registered a meta box inside posts and pages with a check box to prevent that post/page from being shown in the search results. Once the user checks on that check box and hits save/update the post, a meta key called _dont_show_in_search_results with value POST_ID will be saved into the database associated with that post.
Inside pre_get_posts action, I'm doing the following:
function exclude_posts_from_search( $query ){
   if($query->is_main_query() && is_search() ){
     $query->set('meta_query', array(array("key"=>"_dont_show_in_search_results", "compare"=>"NOT EXISTS")));
   }
}

add_action('pre_get_posts','exclude_posts_from_search');

It used to work in the past, but not sure what happened for that query as now it doesn't show any results on the search page!
The WordPress version I'm using is 5.2.3. Seems this issue has been reported many times as bugs, here and here.
If I tried to print the query, it shows like this:
object(WP_Query)[566]
  public 'query' => 
    array (size=1)
      's' => string 'searchTerm' (length=10)
  public 'query_vars' => 
    array (size=53)
      's' => string 'searchTerm' (length=10)
      'error' => string '' (length=0)
      'm' => string '' (length=0)
      'p' => int 0
      'post_parent' => string '' (length=0)
      'subpost' => string '' (length=0)
      'subpost_id' => string '' (length=0)
      'attachment' => string '' (length=0)
      'attachment_id' => int 0
      'name' => string '' (length=0)
      'static' => string '' (length=0)
      'pagename' => string '' (length=0)
      'page_id' => int 0
      'second' => string '' (length=0)
      'minute' => string '' (length=0)
      'hour' => string '' (length=0)
      'day' => int 0
      'monthnum' => int 0
      'year' => int 0
      'w' => int 0
      'category_name' => string '' (length=0)
      'tag' => string '' (length=0)
      'cat' => string '' (length=0)
      'tag_id' => string '' (length=0)
      'author' => string '' (length=0)
      'author_name' => string '' (length=0)
      'feed' => string '' (length=0)
      'tb' => string '' (length=0)
      'paged' => int 0
      'meta_key' => string '' (length=0)
      'meta_value' => string '' (length=0)
      'preview' => string '' (length=0)
      'sentence' => string '' (length=0)
      'title' => string '' (length=0)
      'fields' => string '' (length=0)
      'menu_order' => string '' (length=0)
      'embed' => string '' (length=0)
      'category__in' => 
        array (size=0)
          empty
      'category__not_in' => 
        array (size=0)
          empty
      'category__and' => 
        array (size=0)
          empty
      'post__in' => 
        array (size=0)
          empty
      'post__not_in' => 
        array (size=0)
          empty
      'post_name__in' => 
        array (size=0)
          empty
      'tag__in' => 
        array (size=0)
          empty
      'tag__not_in' => 
        array (size=0)
          empty
      'tag__and' => 
        array (size=0)
          empty
      'tag_slug__in' => 
        array (size=0)
          empty
      'tag_slug__and' => 
        array (size=0)
          empty
      'post_parent__in' => 
        array (size=0)
          empty
      'post_parent__not_in' => 
        array (size=0)
          empty
      'author__in' => 
        array (size=0)
          empty
      'author__not_in' => 
        array (size=0)
          empty
      'meta_query' => 
        array (size=1)
          0 => 
            array (size=2)
              'key' => string '_dont_show_in_search_results' (length=28)
              'compare' => string 'NOT EXISTS' (length=10)
  public 'tax_query' => 
    object(WP_Tax_Query)[1650]
      public 'queries' => 
        array (size=0)
          empty
      public 'relation' => string 'AND' (length=3)
      protected 'table_aliases' => 
        array (size=0)
          empty
      public 'queried_terms' => 
        array (size=0)
          empty
      public 'primary_table' => null
      public 'primary_id_column' => null
  public 'meta_query' => boolean false
  public 'date_query' => boolean false
  public 'post_count' => int 0
  public 'current_post' => int -1
  public 'in_the_loop' => boolean false
  public 'comment_count' => int 0
  public 'current_comment' => int -1
  public 'found_posts' => int 0
  public 'max_num_pages' => int 0
  public 'max_num_comment_pages' => int 0
  public 'is_single' => boolean false
  public 'is_preview' => boolean false
  public 'is_page' => boolean false
  public 'is_archive' => boolean false
  public 'is_date' => boolean false
  public 'is_year' => boolean false
  public 'is_month' => boolean false
  public 'is_day' => boolean false
  public 'is_time' => boolean false
  public 'is_author' => boolean false
  public 'is_category' => boolean false
  public 'is_tag' => boolean false
  public 'is_tax' => boolean false
  public 'is_search' => boolean true
  public 'is_feed' => boolean false
  public 'is_comment_feed' => boolean false
  public 'is_trackback' => boolean false
  public 'is_home' => boolean false
  public 'is_privacy_policy' => boolean false
  public 'is_404' => boolean false
  public 'is_embed' => boolean false
  public 'is_paged' => boolean false
  public 'is_admin' => boolean false
  public 'is_attachment' => boolean false
  public 'is_singular' => boolean false
  public 'is_robots' => boolean false
  public 'is_posts_page' => boolean false
  public 'is_post_type_archive' => boolean false
  private 'query_vars_hash' => string '0d61505a07baa4860f1831a27ad01416' (length=32)
  private 'query_vars_changed' => boolean false
  public 'thumbnails_cached' => boolean false
  private 'stopwords' => null
  private 'compat_fields' => 
    array (size=2)
      0 => string 'query_vars_hash' (length=15)
      1 => string 'query_vars_changed' (length=18)
  private 'compat_methods' => 
    array (size=2)
      0 => string 'init_query_flags' (length=16)
      1 => string 'parse_tax_query' (length=15)

A:

It was a plugin conflict with Cornerstone new search functionality.

