Merge pull request #18670 from craftcms/bugfix/18659-nested-element-duplication

nested element duplication

Author: brandonkelly

CHANGELOG.md

@@ -20,6 +20,7 @@
 - Fixed a bug where the Save button’s spinner wouldn’t appear right away when saving a nested element in a slideout. ([#18664](https://github.com/craftcms/cms/discussions/18664))
 - Fixed a bug where the server check script wasn’t treating GD as a requirement. ([craftcms/server-check#30](https://github.com/craftcms/server-check/issues/30))
 - Fixed a bug where tooltips could be instantiated multiple times within Link fields. ([#18666](https://github.com/craftcms/cms/issues/18666))
+- Fixed a bug where localized nested element content could be overwritten when the owner element was propagated to a new site. ([#18659](https://github.com/craftcms/cms/issues/18659))
 
 ## 5.9.18 - 2026-03-26
 

src/elements/NestedElementManager.php

@@ -966,7 +966,7 @@ private function saveNestedElements(ElementInterface $owner): void
                         } else {
                             // Duplicate the elements, but **don't track** the duplications, so the edit page doesn’t think
                             // its elements have been replaced by the other sites’ nested elements
-                            if ($owner->propagateAll || $this->propagateRequired($owner, $localizedOwner) || !empty($owner->newSiteIds)) {
+                            if ($owner->propagateAll || $this->propagateRequired($owner, $localizedOwner) || in_array($localizedOwner->siteId, $owner->newSiteIds)) {
                                 $this->duplicateNestedElements($owner, $localizedOwner, force: true);
                             }
                         }
@@ -1085,6 +1085,7 @@ public function duplicateNestedElements(
                     'primaryOwner' => $target,
                     'owner' => $target,
                     'propagating' => false,
+                    'resaving' => false,
                     'sortOrder' => $element->getSortOrder(),
                 ];