Hello,
I have checked the products with all the deactivated plugins, and when I tried to filter products by type: Gropped products, nothing appeared, when I activated the plugins and filtered again, two gropped products were found. It means that the grouped products are created by a plugin and they are not Woocommerce grouped product.
Here is the demo of a dropped product on WoodMart theme: https://woodmart.xtemos.com/shop/furniture/wooden-single-drawer/
As you can see everything works correctly. I am checking with Storefront theme and changing the number does not work correctly either: https://gyazo.com/3959e28f322aa7463cd933f9a96d5ed5
The issue is caused by the plugins you use. Our theme supports default Woocommerce products, try to create a dropped product in Woocommerce.
Best Regards