Fall back to "newX" keys for blocks without UUIDs

Resolves #15103

CHANGELOG.md

@@ -16,6 +16,7 @@
 - Fixed a bug where the `users/create` command would fail without explaining why, when the maximum number of users had already been reached.
 - Fixed a validation error that could occur when saving an entry on Craft Solo. ([#15082](https://github.com/craftcms/cms/issues/15082))
 - Fixed an error that could occur on an element edit page, if a Matrix field’s Propagation Method was set to “Custom…”, but its Propagation Key Format wasn’t filled in.
+- Fixed a bug where Matrix block invalidation errors weren’t getting grouped by block when set on the parent element, for blocks that didn’t have `uid` values. ([#15103](https://github.com/craftcms/cms/discussions/15103))
 
 ## 5.1.7 - 2024-05-25
 

src/fields/Matrix.php

@@ -984,6 +984,7 @@ private function validateEntries(ElementInterface $element): void
     {
         /** @var EntryQuery|ElementCollection $value */
         $value = $element->getFieldValue($this->handle);
+        $new = 0;
 
         if ($value instanceof EntryQuery) {
             /** @var Entry[] $entries */
@@ -1004,7 +1005,8 @@ private function validateEntries(ElementInterface $element): void
                 }
 
                 if (!$entry->validate()) {
-                    $element->addModelErrors($entry, "$this->handle[$entry->uid]");
+                    $key = $entry->uid ?? sprintf('new%s', ++$new);
+                    $element->addModelErrors($entry, sprintf('%s[%s]', $this->handle, $key));
                     $allEntriesValidate = false;
                 }
             }