magento2: Invoice Grand Total wrong if discount applied

Preconditions (*)

1. Magento 2.4.1
2. tax/calculation/price_includes_tax => 1
3. tax/calculation/apply_after_discount => 1
4. tax/calculation/discount_tax => 1

Steps to reproduce (*)

1. Create a product with price 20.50 (including tax as per setting above)
2. Tax is set to 19%
3. Create a cart rule for 100% discount
4. Order the product
5. Create invoice

Expected result (*)

1. Order Grand Total is 0
2. Order Tax is 0
3. Invoice Tax is 0
4. Invoice Grand Total excl. Tax is 0
5. Invoice Grand Total incl. Tax is 0

Actual result (*)

1. Order Grand Total is 0
2. Order Tax is 0
3. Invoice Tax is 0
4. Invoice Grand Total excl. Tax is 3.27
5. Invoice Grand Total incl. Tax is 3.27

Probable Reason

In vendor/magento/module-sales/Model/Order/Invoice/Total/Tax.php on line 121 the invoice grand total is set to:

$invoice->setGrandTotal($invoice->getGrandTotal() + $totalTax + $totalDiscountTaxCompensation);

which resolves to:

$invoice->setGrandTotal(0 + 0 + 3.27);

The usage of $totalDiscountTaxCompensation is IMHO wrong when using these settings.

We now have the scenario that the invoices show a grand total > 0 which is not resolvable for accounting since these amounts were never received from the customer.

Basically the same issue occurs if the discount is not 100%, just the amounts differ of course.

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Please provide Severity assessment for the Issue as Reporter. This information will help during Confirmation and Issue triage processes.

• Severity: S0 - Affects critical data or functionality and leaves users without workaround.
• Severity: S1 - Affects critical data or functionality and forces users to employ a workaround.
• Severity: S2 - Affects non-critical data or functionality and forces users to employ a workaround.
• 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”.