Display only posts with thumbnails

Question

I'm trying to have a WP theme display only posts that have a thumbnail.

Below is the code of the file that shows some posts on the frontpage. I've already tried to change the code (please see the //added by me start/stop) and from what I've read it should do the trick. However, on the front page, posts without thumbnails are displayed as well.

<?php
if ( ! get_theme_mod( 'flash_news_enable_posts_list_section', false ) ) {
    return;
}

$content_ids   = array();
$content_count = get_theme_mod( 'flash_news_posts_list_count', 6 );
$content_type  = get_theme_mod( 'flash_news_posts_list_content_type', 'category' );

if ( in_array( $content_type, array( 'post', 'page' ) ) ) {

    for ( $i = 1; $i <= $content_count; $i++ ) {
        $content_ids[] = get_theme_mod( 'flash_news_posts_list_content_' . $content_type . '_' . $i );
    }

    $args = array(
        'post_type'           => $content_type,
        'post__in'            => array_filter( $content_ids ),
        'orderby'             => 'post__in',
        'posts_per_page'      => absint( $content_count ),
        'ignore_sticky_posts' => true,
//added by me start
    'meta_query' => array(
        array(
         'key' => '_thumbnail_id',
         'compare' => 'EXISTS'
        ),
    ),
//added by me stop
    );

} elseif ( $content_type === 'category' ) {
    $cat_content_id = get_theme_mod( 'flash_news_posts_list_content_category' );
    $args           = array(
        'cat'            => $cat_content_id,
        'posts_per_page' => absint( $content_count ),
//added by me start
    'meta_query' => array(
        array(
         'key' => '_thumbnail_id',
         'compare' => 'EXISTS'
        ),
    ),
//added by me stop
    );
} elseif ( $content_type === 'recent' ) {
    $args = array(
        'post_type'           => 'post',
        'posts_per_page'      => absint( $content_count ),
        'ignore_sticky_posts' => true,
//added by me start
    'meta_query' => array(
        array(
         'key' => '_thumbnail_id',
         'compare' => 'EXISTS'
        ),
    ),
//added by me stop
    );
}

$args = apply_filters( 'flash_news_posts_list_section_args', $args );

flash_news_render_posts_list_section( $args );

/**
 * Render Posts List Section.
 */
function flash_news_render_posts_list_section( $args ) {
    $section_title = get_theme_mod( 'flash_news_posts_list_title', __( 'Posts List', 'flash-news' ) );
    $button_label  = get_theme_mod( 'flash_news_posts_list_button_label', __( 'View All', 'flash-news' ) );
    $button_link   = get_theme_mod( 'flash_news_posts_list_button_link' );
    $button_link   = ! empty( $button_link ) ? $button_link : '#';
    $tile_style    = get_theme_mod( 'flash_news_posts_list_section_style', 'style-1' );

    $query = new WP_Query( $args );
    if ( $query->have_posts()) :
        ?>
        <section id="flash_news_posts_list_section" class="magazine-frontpage-section magazine-small-list-section <?php echo esc_attr( $tile_style ); ?>">
            <?php
            if ( is_customize_preview() ) :
                flash_news_section_link( 'flash_news_posts_list_section' );
            endif;
            ?>
            <div class="ascendoor-wrapper">
                <div class="section-header">
                    <h3 class="section-title"><?php echo esc_html( $section_title ); ?></h3>
                    <a href="<?php echo esc_url( $button_link ); ?>" class="mag-view-all-link"><?php echo esc_html( $button_label ); ?></a>
                </div>
                <div class="magazine-section-body">
                    <div class="magazine-list-section-wrapper">
                        <?php
                        while ( $query->have_posts() ) :
                            $query->the_post();
                            ?>
                            <div class="mag-post-single has-image list-design">
                                <div class="mag-post-img">
                                    <a href="<?php the_permalink(); ?>">
                                        <?php the_post_thumbnail(); ?>
                                    </a>
                                </div>
                                <div class="mag-post-detail">
                                    <div class="mag-post-detail-inner">
                                        <div class="mag-post-category">
                                            <?php flash_news_categories_list(); ?>
                                        </div>
                                        <h3 class="mag-post-title">
                                            <a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
                                        </h3>
                                        <div class="mag-post-meta">
                                            <span class="post-author">
                                                <a href="<?php echo esc_url( get_author_posts_url( get_the_author_meta( 'ID' ) ) ); ?>"><i class="fas fa-user"></i><?php echo esc_html( get_the_author() ); ?></a>
                                            </span>
                                            <span class="post-date">
                                                <a href="<?php the_permalink(); ?>"><i class="far fa-clock"></i><?php echo esc_html( get_the_date() ); ?></a>
                                            </span>
                                        </div>
                                    </div>
                                </div>
                            </div>
                            <?php
                        endwhile;
                        wp_reset_postdata();
                        ?>
                    </div>
                </div>
            </div>
        </section>
        <?php
    endif;

}

What could be wrong?