Hi,
Please stay on topic and open a new thread for unrelated issues.
You're saying that a discount apply or not based on the number of categories of a product.
However, the ordered quantity of a product doesn't take any account of the number of products in the cart.
That's only for coupons. Discounts will apply regardless of that.
So please, when you open the new thread, provide a screenshot of the settings of your discount (coupon ?) and a screenshot of how the product is configured when the problem happens so that we can have a clear picture of the situation.