ACP2E-3349: Cart rules "Fixed amount discount for whole cart" Action applies discounts incorrectly

Author: aplapana

app/code/Magento/SalesRule/Model/Quote/Discount.php

@@ -188,7 +188,7 @@ public function collect(
         $items = $this->calculator->sortItemsByPriority($items, $address);
         $itemsToApplyRules = $items;
         $rules = $this->calculator->getRules($address);
-        $totalDiscount = 0;
+        $totalDiscount = [];
         $address->setBaseDiscountAmount(0);
         /** @var Rule $rule */
         foreach ($rules as $rule) {
@@ -221,9 +221,9 @@ public function collect(
                     unset($itemsToApplyRules[$key]);
                 }
 
-                $totalDiscount += $item->getBaseDiscountAmount();
+                $totalDiscount[$item->getId()] = $item->getBaseDiscountAmount();
             }
-            $address->setBaseDiscountAmount($totalDiscount);
+            $address->setBaseDiscountAmount(array_sum(array_values($totalDiscount)));
         }
         $this->calculator->initTotals($items, $address);
         foreach ($items as $item) {