Support Forum

Those are the query args which don’t include the value for the featured image.

You need to filter the featured image directly, which is more complicated.

For example:

add_filter( 'post_thumbnail_html', function( $html, $post->ID, $post_thumbnail_id, $size, $attr ) {
    $custom_field = get_post_meta( get_the_ID(), 'your_custom_field', true );

    if ( '' == $html && $custom_field ) {
        $html = '<img src="' . $custom_field . '" alt="" />';
    }

    return $html;
}, 10, 5 );