Incorrect sequence values in INSERT SELECT

SELECT VERSION();

DROP TABLE IF EXISTS `t1`;

DROP SEQUENCE IF EXISTS `s1`;

CREATE SEQUENCE IF NOT EXISTS `s1`;

CREATE TABLE IF NOT EXISTS `t1` (

  `a` BIGINT UNSIGNED NOT NULL PRIMARY KEY

      DEFAULT (NEXT VALUE FOR `s1`),

  `b` CHAR(1) NOT NULL

) ENGINE=InnoDB;

SHOW CREATE TABLE `s1`;

INSERT INTO `t1` (`b`) VALUES ('a');

INSERT INTO `t1` (`b`) VALUES ('b'), ('c');

INSERT INTO `t1` (`b`) VALUES ('d');

INSERT INTO `t1` (`b`)

SELECT `c`

FROM (

  SELECT 'e' `c`

  UNION

  SELECT 'f' `c`

  UNION

  SELECT 'g' `c`

) `der`;

SELECT

  `a`, `b`

FROM

  `t1`;