Index: src/wp-includes/class-wp-block-parser.php
===================================================================
--- src/wp-includes/class-wp-block-parser.php	(revision 46560)
+++ src/wp-includes/class-wp-block-parser.php	(working copy)
@@ -288,7 +288,8 @@
 				 * have to assume that multiple closers are missing
 				 * and so we'll collapse the whole stack piecewise
 				 */
-				while ( 0 < count( $this->stack ) ) {
+				$stack_count = count( $this->stack );
+				while ( 0 < $stack_count ) {
 					$this->add_block_from_stack();
 				}
 				return false;