Discount Qty Applies to Mixed Attributes doesn't work

Just noticed the "Discount Qty Applies to Mixed Attributes" function doesn't work in v1.5.8a

Either "Yes" or "No" gives me the same incorrect result (no discount qty applied). I tested with PHP 7.4 and 8.0, but still didn't get the correct amount. Is there any possible setting conflict?

This works in master. Not sure if it was fixed in 2.0.x or 2.1.0.

Looks good in 2.0.1 as well.

It looks like a similar issue from https://www.zen-cart.com/showthread.php?230034-Products-Price-Manager-discounts-aren-t-working-on-multiple-attributes-even-though-th

So I copy the following codes (from v2.0.1) to \includes\functions\functions_price.php and insert between line 1417 and 1418:

if (IS_ADMIN_FLAG === false) {
$new_qty = $_SESSION['cart']->in_cart_mixed_discount_quantity($product_id);
// check for discount qty mix
if ($new_qty > $check_qty) {
$check_qty = $new_qty;
}
}

So far it works fine, and no error logs generated.
I would appreciate it if anyone could look into and verify my modification.