Hello,
The thing is, according to the default working of the Variable Product under the WooCommerce, if you have defined two Attributes for a Product and using them as Variants then to make the Product add to cart both of them needs to be selected, even if any of the Variant value of an Attribute is separately defined too. So, this is how it works for the Variable Products.
You can once check your concern with the default WordPress Theme activated too to confirm it more and if you find any difference then mention us so that we can further look into this.
Best Regards