diff --git a/src/wp-includes/block-template-utils.php b/src/wp-includes/block-template-utils.php
index b8538ee3b43e2..360d2419b4979 100644
--- a/src/wp-includes/block-template-utils.php
+++ b/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;
 }
 
diff --git a/src/wp-includes/blocks.php b/src/wp-includes/blocks.php
index 4d3a34e34fa77..c92ea78562f8a 100644
--- a/src/wp-includes/blocks.php
+++ b/src/wp-includes/blocks.php
@@ -794,16 +794,22 @@ function get_comment_delimited_block_content( $block_name, $block_attributes, $b
  * instead preserve the markup as parsed.
  *
  * @since 5.3.1
+ * @since 6.4.0 The `$callback` parameter was added.
  *
  * @param array $block A representative array of a single parsed block object. See WP_Block_Parser_Block.
+ * @param string $callback Optional. Callback to run on the block before serialization.
  * @return string String of rendered HTML.
  */
-function serialize_block( $block ) {
+function serialize_block( $block, $callback = null ) {
+	if ( is_callable( $callback ) ) {
+		$block = call_user_func( $callback, $block );
+	}
+
 	$block_content = '';
 
 	$index = 0;
 	foreach ( $block['innerContent'] as $chunk ) {
-		$block_content .= is_string( $chunk ) ? $chunk : serialize_block( $block['innerBlocks'][ $index++ ] );
+		$block_content .= is_string( $chunk ) ? $chunk : serialize_block( $block['innerBlocks'][ $index++ ], $callback );
 	}
 
 	if ( ! is_array( $block['attrs'] ) ) {