Hi,
On listings, if you have the display of the add to cart button activated, HikaShop will display it if possible.
There are several reasons which make it impossible. And in these cases, it will display a "Choose options" button instead which will direct the user to the product details page.
For example, if you have variants for a product, then the user has to be directed to the product page where he will be able to choose the variant before adding it to the cart.
Another example is if the product has options.
And a last possibility is if the product has custom fields of the table "item" which are required and the display of the custom item fields on the listing is not activated.
Since you're saying that based on the assigned category, you get the choose button or the add to cart button, I suppose that you must have custom item fields restricted based on the "categories" setting. And there must be something linked to that.
I think we would need a backend and a FTP access in order to check the situation.