diff -u b/tests/src/Kernel/Processor/AddHierarchyTest.php b/tests/src/Kernel/Processor/AddHierarchyTest.php
--- b/tests/src/Kernel/Processor/AddHierarchyTest.php
+++ b/tests/src/Kernel/Processor/AddHierarchyTest.php
@@ -273,6 +273,11 @@
       ]);
     }
 
+    // Also add a term with multiple parents.
+    $this->terms['avocado'] = $this->createTerm($this->vocabulary, [
+      'name' => 'Avocado',
+      'parent' => [$this->terms['fruit']->id(), $this->terms['vegetable']->id()],
+    ]);
     $this->nodes[3] = $this->createNode([
       'type' => 'page',
       'term_field' => [