Hi,
HikaShop have some restriction features like an option to forbidden to buy free products.
As expected, the option "Display 'add to cart' button for free products" was set to no.
So you can't add free products in the cart.
Before the options are not added directly, you can see them added but they are removed during the next check.
Otherwise, you can use the setting "Group options with product" to group the options to the main product. It should avoid the price check.
So you had some restrictions.
Regards,