Fix aborted rendering of non-nested reusable blocks

Since #28405, `render_block_core_block` prevents fatal rendering loops
in Reusable Blocks by aborting if a given block has already been
rendered. This effectively prevents loops, but also unintentionally
prevented the same reusable block from being rendered twice on the same
page, even if one isn't nested in the other.

This commit fixes this by explicitly forgetting a Reusable Block after
it has finished rendering.

packages/block-library/src/block/index.php

@@ -24,7 +24,7 @@ function render_block_core_block( $attributes ) {
 		return '';
 	}
 
-	if ( in_array( $attributes['ref'], $seen_refs, true ) ) {
+	if ( isset( $seen_refs[ $attributes['ref'] ] ) ) {
 		if ( ! is_admin() ) {
 			trigger_error(
 				sprintf(
@@ -47,13 +47,15 @@ function render_block_core_block( $attributes ) {
 			'';
 	}
 
-	$seen_refs[] = $attributes['ref'];
-
 	if ( 'publish' !== $reusable_block->post_status || ! empty( $reusable_block->post_password ) ) {
 		return '';
 	}
 
-	return do_blocks( $reusable_block->post_content );
+	$seen_refs[ $attributes['ref'] ] = true;
+
+	$result = do_blocks( $reusable_block->post_content );
+	unset( $seen_refs[ $attributes['ref'] ] );
+	return $result;
 }
 
 /**