Custom product fields only work at variable products and not simple

Hi, the plugin only seems to work with Variable products. With simpel products the custom products show, but can’t be saved (see cart)

Simpel product (doesn’t work): https://liabox.ran-e-demo.nl/product/luxe-kerstchocolade-truffels-bonbons-stang-kerst-truffels-bonbons-7st/

Variable Product(works): https://liabox.ran-e-demo.nl/product/chocolade-reep-bedankt-melk-puur-of-wit-90g/

The page I need help with: [log in to see the link]