[Mappings editor] Fix bug when switching between mapping tabs

x-pack/plugins/index_management/public/application/components/mappings_editor/__jest__/client_integration/mappings_editor.test.tsx

@@ -223,6 +223,33 @@ describe('Mappings editor: core', () => {
       isNumericDetectionVisible = exists('advancedConfiguration.numericDetection');
       expect(isNumericDetectionVisible).toBe(false);
     });
+
+    test('should keep default dynamic templates value when switching tabs', async () => {
+      await act(async () => {
+        testBed = setup({
+          value: { ...defaultMappings, dynamic_templates: [] }, // by default, the UI will provide an empty array for dynamic templates
+          onChange: onChangeHandler,
+        });
+      });
+      testBed.component.update();
+
+      const {
+        actions: { selectTab, getJsonEditorValue },
+      } = testBed;
+
+      // Navigate to dynamic templates tab and verify empty array
+      await selectTab('templates');
+      let templatesValue = getJsonEditorValue('dynamicTemplatesEditor');
+      expect(templatesValue).toEqual([]);
+
+      // Navigate to advanced tab
+      await selectTab('advanced');
+
+      // Navigate back to dynamic templates tab and verify empty array persists
+      await selectTab('templates');
+      templatesValue = getJsonEditorValue('dynamicTemplatesEditor');
+      expect(templatesValue).toEqual([]);
+    });
   });
 
   describe('component props', () => {

x-pack/plugins/index_management/public/application/components/mappings_editor/components/templates_form/templates_form.tsx

@@ -36,11 +36,10 @@ const formSerializer: SerializerFunc<MappingsTemplates | undefined> = (formData)
     // Silently swallow errors
   }
 
-  return Array.isArray(parsedTemplates) && parsedTemplates.length > 0
-    ? {
-        dynamic_templates: parsedTemplates,
-      }
-    : undefined;
+  return {
+    dynamic_templates:
+      Array.isArray(parsedTemplates) && parsedTemplates.length > 0 ? parsedTemplates : [],
+  };
 };
 
 const formDeserializer = (formData: { [key: string]: any }) => {