ENGCOM-3450: [Forwardport] Fixed tierprice discount not calculated correctly if has specialprice #19179

 - Merge Pull Request #19179 from torhoehn/magento2:2.3-develop-PR-port-18743
 - Merged commits:
   1. 8f463db
   2. 63c9ea4
   3. 77492c1
   4. d0ba997

Author: magento-engcom-team

app/code/Magento/Catalog/Model/Product/Attribute/Backend/Tierprice.php

@@ -159,8 +159,22 @@ protected function validatePrice(array $priceRow)
      */
     protected function modifyPriceData($object, $data)
     {
+        /** @var \Magento\Catalog\Model\Product $object */
         $data = parent::modifyPriceData($object, $data);
         $price = $object->getPrice();
+
+        $specialPrice = $object->getSpecialPrice();
+        $specialPriceFromDate = $object->getSpecialFromDate();
+        $specialPriceToDate = $object->getSpecialToDate();
+        $today = time();
+
+        if ($specialPrice && ($object->getPrice() > $object->getFinalPrice())) {
+            if ($today >= strtotime($specialPriceFromDate) && $today <= strtotime($specialPriceToDate) ||
+                $today >= strtotime($specialPriceFromDate) && $specialPriceToDate === null) {
+                $price = $specialPrice;
+            }
+        }
+
         foreach ($data as $key => $tierPrice) {
             $percentageValue = $this->getPercentage($tierPrice);
             if ($percentageValue) {