Can customers buy subscription only product as a single product?

Hi, can customers buy products that have “Only allow purchase of this product as a subscription” checked as a single product.

Because at shop using our subscription app, some orders related to subscription only product is bought as a single product.

we found that even if “Only allow purchase of this product as a subscription” is checked, if the customer click the checkout button without selling_id and proceed to checkout, the customer can buy as a single product.

is this a bug?