Index: src/wp-includes/blocks.php
===================================================================
--- src/wp-includes/blocks.php	(revision 43938)
+++ src/wp-includes/blocks.php	(working copy)
@@ -237,6 +237,7 @@
  * @return string Updated post content.
  */
 function do_blocks( $content ) {
+	$content = ltrim($content, '<!-- /wp:nextpage -->');
 	// If there are blocks in this content, we shouldn't run wpautop() on it later.
 	$priority = has_filter( 'the_content', 'wpautop' );
 	if ( false !== $priority && doing_filter( 'the_content' ) && has_blocks( $content ) ) {