Inject theme attribute during serialization

src/wp-includes/block-template-utils.php

@@ -513,6 +513,26 @@ function _inject_theme_attribute_in_block_template_content( $template_content )
 	return $template_content;
 }
 
+/**
+ * Injects the active theme's stylesheet as a `theme` attribute
+ * into a given template part block.
+ *
+ * @since 6.4.0
+ * @access private
+ *
+ * @param array $block a parsed block.
+ * @return array Updated block.
+ */
+function _inject_theme_attribute_in_block_template_block( $block ) {
+	if (
+		'core/template-part' === $block['blockName'] &&
+		! isset( $block['attrs']['theme'] )
+	) {
+		$block['attrs']['theme'] = get_stylesheet();
+	}
+	return $block;
+}
+
 /**
  * Parses a block template and removes the theme attribute from each template part.
  *
@@ -565,7 +585,6 @@ function _build_block_template_result_from_file( $template_file, $template_type
 	$template                 = new WP_Block_Template();
 	$template->id             = $theme . '//' . $template_file['slug'];
 	$template->theme          = $theme;
-	$template->content        = _inject_theme_attribute_in_block_template_content( $template_content );
 	$template->slug           = $template_file['slug'];
 	$template->source         = 'theme';
 	$template->type           = $template_type;
@@ -589,6 +608,9 @@ function _build_block_template_result_from_file( $template_file, $template_type
 		$template->area = $template_file['area'];
 	}
 
+	$blocks            = parse_blocks( $template_content );
+	$template->content = serialize_blocks( $blocks, '_inject_theme_attribute_in_block_template_block' );
+
 	return $template;
 }