Bug on related products after update 2.3.4

Hi, related products disappeared from product page of some products after last update.

Please have a look here, there are no related products http://www.tattoodefender.com/en/products/sunny-side/

If you add product to cart then strangely related products show.

On this product instead related products are correctly shown by default http://www.tattoodefender.com/prodotti/after-ink/

I did not changed anything on my backend or products settings, only updated plugin.

https://wordpress.org/plugins/woocommerce/