ENGCOM-3158: Fix for custom product attribute changing 'backend_type'…

… when 'is_user_defined = 1' and get updated/saved in Admin Backend #18196

 - Merge Pull Request #18196 from bartoszkubicki/magento2:attribute-backend-type-unexpected-change
 - Merged commits:
   1. f6a9459

app/code/Magento/Catalog/Controller/Adminhtml/Product/Attribute/Save.php

@@ -222,14 +222,14 @@ public function execute()
                 $data['backend_model'] = $this->productHelper->getAttributeBackendModelByInputType(
                     $data['frontend_input']
                 );
+
+                if ($model->getIsUserDefined() === null) {
+                    $data['backend_type'] = $model->getBackendTypeByInput($data['frontend_input']);
+                }
             }
 
             $data += ['is_filterable' => 0, 'is_filterable_in_search' => 0];
 
-            if ($model->getIsUserDefined() === null || $model->getIsUserDefined() != 0) {
-                $data['backend_type'] = $model->getBackendTypeByInput($data['frontend_input']);
-            }
-
             $defaultValueField = $model->getDefaultValueByInput($data['frontend_input']);
             if ($defaultValueField) {
                 $data['default_value'] = $this->getRequest()->getParam($defaultValueField);