I don’t know if you’re still looking for a solution, but you can use the post_thumbnail_html filter in you php file to return a fallback; like so.
/**
* Set the default image if none exists.
*
* @param string $html The post thumbnail HTML.
* @param int $post_id The post ID.
* @param int $post_thumbnail_id The post thumbnail ID.
* @return html
*/
public function wp_893874_fallback_post_thumbnail_html( $html, $post_id, $post_thumbnail_id ) {
if ( empty( $html ) ) {
$html="<img src="http://lorempixel.com/g/400/200" width="400" height="200" loading="lazy" alt="" . get_the_title( $post_id ) . '" />';
}
return $html;
}
add_filter( 'post_thumbnail_html', 'wp_893874_fallback_post_thumbnail_html', 5, 3 );