Hi @kdjmonaghan. Thank you for your report.
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For users who seek a fix until Magento either make an option for this in an update, or, make a merge request for this;

Your discount_description field is being populated with the Coupon's description now where as previously in older versions of Magento it was using coupon_code and never actually checked for the presence of the Coupon's description to populate this field.

In recent builds of Magento, Magento have 'fixed' this issue (causing your legacy orders invoices contents data to not match the expected result of new orders, where your legacy orders were 'correct' before) - this results in the coupon code used not being present anywhere to the normal Admin user who does not know how to look up the Coupon Code used within the Order, easily.

The following file is the issue within Vendor:

vendor/magento/module-sales-rule/Model/RulesApplier.php

Look for the addDiscountDescription method and you will see there is a new condition added for setting the Coupon Code Description to the $label variable after the getCouponCode check.

Previously this method, addDiscountDescription looked like this, and you're going to have to replace it via an Extension or Plugin to get it to work as it did before. Comparing the below to what's in this file now will show you the change:

    public function addDiscountDescription($address, $rule)
    {
        $description = $address->getDiscountDescriptionArray();
        $ruleLabel = $rule->getStoreLabel($address->getQuote()->getStore());
        $label = '';
        if ($ruleLabel) {
            $label = $ruleLabel;
        } else {
            if (strlen($address->getCouponCode())) {
                $label = $address->getCouponCode();
            }
        }

        if (strlen($label)) {
            $description[$rule->getId()] = $label;
        }

        $address->setDiscountDescriptionArray($description);

        return $this;
    }

You're going to have to either make a Plugin or extend this module and replace this method within the class to have the original functionality work as the way it did in previous releases of Magento 2.

Your invoices templates, by default, aren't expecting, or formatted, to handle a long description that you can enter into the Discount Description field. This may have been patched as a 'long term bug' (it probably was intended to work like this) but can cause frustration for those expeccting the base functionality to work as it has in previous iterations.

Hi @krishprakash. Thank you for working on this issue.
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@magento give me 2.4-develop instance

Hi @krishprakash. Thank you for your request. I'm working on Magento 2.4-develop instance for you

Hi @krishprakash, here is your Magento instance.
Admin access: https://i-28432-2-4-develop.instances.magento-community.engineering/admin_043f
Login: 4087a0e1 Password: 86b17b7cf138
Instance will be terminated in up to 3 hours.

Hi @engcom-Alfa. Thank you for working on this issue.
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Hi @engcom-Bravo. Thank you for working on this issue.
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@magento give me 2.4-develop instance

Hi @engcom-Bravo. Thank you for your request. I'm working on Magento instance for you.

Hi @engcom-Bravo, here is your Magento Instance: https://7353e34217821129194bf61186089f6e.instances-prod.magento-community.engineering
Admin access: https://7353e34217821129194bf61186089f6e.instances-prod.magento-community.engineering/admin_d78a
Login: bad13834
Password: 8f08dc148309

HI @kdjmonaghan,

Thanks for your reporting and collaboration.

We have verified the issue in Latest 2.4-develop instance and the issue is reproducible.Kindly refer the screenshots.

Coupon Code is missing from Order, Invoice and Invoice PDF as Discount Description column is now being populated with the description field and not the invoice when Description is populated.

Hence Confirming the issue.

Thanks.

✅ Jira issue https://jira.corp.adobe.com/browse/AC-11991 is successfully created for this GitHub issue.

✅ Confirmed by @engcom-Bravo. Thank you for verifying the issue.
Issue Available: @engcom-Bravo, You will be automatically unassigned. Contributors/Maintainers can claim this issue to continue. To reclaim and continue work, reassign the ticket to yourself.