Placeholder-thumbnails shown.

I tried the patch at https://ww.wp.xz.cn/support/topic/handling-products-with-no-thumbnail/

However, perhaps because the products were added by api, all products have thumbnails, if product didn’t have an image, it got a placeholder: /wp-content/plugins/woocommerce/assets/images/placeholder.png

The filter does not catch these placeholder-thumbnails different from real images.