Index: src/wp-includes/post-template.php
===================================================================
--- src/wp-includes/post-template.php	(revision 44830)
+++ src/wp-includes/post-template.php	(working copy)
@@ -297,6 +297,10 @@
 	$content = $pages[ $page - 1 ];
 	if ( preg_match( '/<!--more(.*?)?-->/', $content, $matches ) ) {
 		$content = explode( $matches[0], $content, 2 );
+
+		// Remove the core/more block tags now that it's been split up.
+		$content = preg_replace( '/<!-- \/?wp:more(.*?) -->/', '', $content );
+
 		if ( ! empty( $matches[1] ) && ! empty( $more_link_text ) ) {
 			$more_link_text = strip_tags( wp_kses_no_null( trim( $matches[1] ) ) );
 		}