We should push and pop

src/wp-includes/html-api/class-wp-html-processor.php

@@ -950,6 +950,7 @@ private function step_in_body() {
 					$this->close_a_p_element();
 				}
 				$this->insert_html_element( $this->state->current_token );
+				$this->state->stack_of_open_elements->pop();
 				$this->state->frameset_ok = false;
 				return true;
 		}