woocommerce_coupon_get_discount_amount returned value applied once per item, not once per cart

Question

I am using filter woocommerce_coupon_get_discount_amount filter hook to recalculate coupon discount based on product attributes. Code calculates the result I want, but the returned value is applied at the cart to each line item, I want it to apply as a single discount to the whole cart.

For example, if I calculate and return $100, but there are 4 items in the cart, the discount on the cart becomes $400.

Am I using the correct filter for cart discount? Is there a better way to apply the discount only to items with certain product attributes?

The discount should only apply to items with size of 'Low-Res'.

/* apply 'free download' coupon to 'Low-Res' download items only */
/* by calculating their qty and price and updating the discount */
add_filter( 'woocommerce_coupon_get_discount_amount', 'filter_woocommerce_coupon_get_discount_amount', 10, 1 );
function filter_woocommerce_coupon_get_discount_amount( $discounting_amount ) {    

    $discount_is_for = 'Low-Res';
    $coupon_code = 'free download';
    $discounting_amount = 0;

    $cart_coupons = WC()->cart->get_coupons();
    
    // check to see if the 'free download' coupon has been added
    foreach ($cart_coupons as $coupon) {
        if ($coupon->code == $coupon_code) {
            foreach ( WC()->cart->get_cart() as $cart_item ) {
                
                // check to see if it's a Low-Res download
                $product = $cart_item['data'];
                $attributes = $product->get_attributes();
                if ($product->get_attribute( 'pa_size' ) == $discount_is_for) {
                    
                    // gets the cart item quantity
                    $quantity           = $cart_item['quantity'];
                    // gets the cart item subtotal
                    $line_subtotal      = $cart_item['line_subtotal']; 
                    $line_subtotal_tax  = $cart_item['line_subtotal_tax'];
                    // gets the cart item total
                    $line_total         = $cart_item['line_total'];
                    $line_tax           = $cart_item['line_tax'];
                    // unit price of the product
                    $item_price         = $line_subtotal / $quantity;
                    $item_tax           = $line_subtotal_tax / $quantity;
                    
                    $discounting_amount += $cart_item['quantity'] * ($item_price + $item_tax);
                }
            }
        }
    }
    
    return $discounting_amount;
}