ENGCOM-5525: [Catalog] Fix user value verification when adding product with multiselect custom option to cart #23871

 - Merge Pull Request #23871 from magenius-team/magento2:bug/multiselect-options-verification
 - Merged commits:
   1. 1acde36

Author: magento-engcom-team

app/code/Magento/Catalog/Model/Product/Option/Type/Select.php

@@ -84,6 +84,9 @@ public function validateUserValue($values)
             );
         }
         if (!$this->_isSingleSelection()) {
+            if (is_string($value)) {
+                $value = explode(',', $value);
+            }
             $valuesCollection = $option->getOptionValuesByOptionId($value, $this->getProduct()->getStoreId())->load();
             $valueCount = is_array($value) ? count($value) : 0;
             if ($valuesCollection->count() != $valueCount) {