100% Discount cart rule results in Tax invoiced

[OldPlanets]

Preconditions (*)

1. Magento 2.3.6
2. Magento 2.4-develop
3. Free shipping is enabled

Steps to reproduce (*)

1. Navigate to Stores / Settings / Configuration / Sales / Tax
2. Set Catalog Prices and Apply Discount on Prices to Including Tax
   
3. Navigate to Sores / Tax Rules / Add New Tax Rule
4. Create a rule for all USA with 10% tax rate and apply it
5. Navigate to Marketing / Cart Price Rules and Add New Rule
6. Create a rule with 100% discount for all users
   
7. Make an order on Storefront:

• choose Free Shipping
• apply coupon code

8. Navigate to Sales / Orders and open your order
9. Create an invoice for the order and open it

Expected result (*)

1. An invoice is created over 0,00$

Actual result (*)

1. An invoice with the Grand Total of the tax amount is created
   
   

I'm suspect this bug was introduced in this commit (MC-35675):
2dbab41

It doesn't allow GrandTotals to go below zero, but the Tax Totals will add the discounted Tax, expecting that it was deducted before, resulting in a Invoice which is never smaller than the tax amount (instead of being 0)

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• Severity: S1 - Affects critical data or functionality and forces users to employ a workaround.
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• Severity: S3 - Affects non-critical data or functionality and does not force users to employ a workaround.
• Severity: S4 - Affects aesthetics, professional look and feel, “quality” or “usability”.

[m2-assistant]

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[m2-assistant]

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[magento-engcom-team]

✅ Confirmed by @engcom-Delta
Thank you for verifying the issue. Based on the provided information internal tickets MC-39052 were created

Issue Available: @engcom-Delta, You will be automatically unassigned. Contributors/Maintainers can claim this issue to continue. To reclaim and continue work, reassign the ticket to yourself.

[paulvandermeijs]

I noticed in app/code/Magento/Sales/Model/Order/Invoice/Total/Tax.php that the discount tax compensation is added to the grand total; if I'm right that is the amount that is removed from the tax by a discount. In that case shouldn't it be excluded from the grand total?

magento2/app/code/Magento/Sales/Model/Order/Invoice/Total/Tax.php

Lines 121 to 122 in 445b0f1

	$invoice->setGrandTotal($invoice->getGrandTotal() + $totalTax + $totalDiscountTaxCompensation);
	$invoice->setBaseGrandTotal($invoice->getBaseGrandTotal() + $baseTotalTax + $baseTotalDiscountTaxCompensation);

[ThomasNegeli]

@paulvandermeijs
Until 2.2.11 the $invoice->getGrandTotal() was a negative value in that case.
In 2.3 something happenend so that the $invoice->getGrandTotal() returns 0 (which is logically right, but wrong when adding tax compensation).