I have a grouped product that still displays the ‘Add to cart’ button when one of the products where the grouped product is made out of is out of stock. On the single product the ‘Add to cart’ button is hidden and works fine, but the grouped product is still displaying a button.

Single product: https://www.b4t.impleodev.nl/winkel/12345/
Grouped product: https://www.b4t.impleodev.nl/winkel/robotmaaier-messen-husqvarna-36-stuks/

How can I hide the button on the grouped product as well when out of stock?