Percentage discounts using cart price rules being calculated incorrectly

[jamie-selesti]

Preconditions (*)

1. Magento 2.3.6-p1
2. PHP 7.3

Steps to reproduce (*)

1. Set discounts to be applied before tax in the admin
2. Set up a cart price rule for 15% ( Apply Percent of product price discount)
3. Add multiple of products into your basket and apply the coupon code
4. Checkout & pay

For reference, this is how tax is set up in the admin

Expected result (*)

1. For the 15% to be consistent across all products and not to have any rounding issue

Actual result (*)

1. As you can see from the image below the logic in Magento is not correct as sometimes its 1p out

This is a screenshot from the Magento admin with the incorrect values highlighted

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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.
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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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[jamie-selesti]

As an update, if I comment out /vendor/magento/module-sales-rule/Model/RulesApplier.php:217 which is

//$this->validatorUtility->deltaRoundingFix($discountData, $item);

Then all the maths and logic then adds up as expected so I think the bug / issue is connected to the DeltaRoundingFix

[m2-assistant]

Hi @engcom-Oscar. Thank you for working on this issue.
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[magento-engcom-team]

✅ Confirmed by @engcom-Oscar
Thank you for verifying the issue. Based on the provided information internal tickets MC-41350 were created

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

[doninAtwix]

@magento I am working on this

[LuisLiaw7155]

any update for this?