diff -u b/core/modules/migrate/src/Row.php b/core/modules/migrate/src/Row.php
--- b/core/modules/migrate/src/Row.php
+++ b/core/modules/migrate/src/Row.php
@@ -106,9 +106,13 @@
    * Retrieves the values of the source identifiers.
    *
    * @return array
-   *   An array containing the values of the source identifiers.
+   *   An array containing the values of the source identifiers. Returns values
+   *   in the same order as defined in $this->sourceIds.
    */
   public function getSourceIdValues() {
+    // First, return the array of keys properly ordered with
+    // array_flip(array_keys()). Then, fill in values by merging with
+    // array_intersect_key().
     return array_merge(array_flip(array_keys($this->sourceIds)), array_intersect_key($this->source, $this->sourceIds));
   }