Resolve #3506821 "Referencing multiple"

Closes #3506821

src/AiReferenceGenerator.php

@@ -262,6 +262,19 @@ class AiReferenceGenerator {
           unset($options[$existing_value['target_id']]);
         }
       }
+
+      // Get settable options can return options group by bundle type if there
+      // are multiple target bundles.
+      if ($options) {
+        $first_option = reset($options);
+        if (is_array($first_option)) {
+          $merged_options = [];
+          foreach ($options as $option_group) {
+            $merged_options += $option_group;
+          }
+          return $merged_options;
+        }
+      }
       return $options;
     }
     return [];