Cannot disable FOREIGN_KEY_CHECKS anymore for converting character set

CREATE TABLE `t1` (

  `hash` varchar(32) NOT NULL COLLATE 'utf8_unicode_ci',

  `title` varchar(100) DEFAULT NULL COLLATE 'utf8_unicode_ci',

  PRIMARY KEY (`hash`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;	

CREATE TABLE `t2` (

  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,

  `t1_hash` varchar(32) NOT NULL  COLLATE 'utf8_unicode_ci',

  `name` varchar(100) DEFAULT NULL  COLLATE 'utf8_unicode_ci',

  PRIMARY KEY (`id`),

  KEY `FK_t1_hash` (`t1_hash`),

  CONSTRAINT `FK_t1_hash` FOREIGN KEY (`t1_hash`) REFERENCES `t1` (`hash`) ON DELETE CASCADE ON UPDATE NO ACTION

) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

SET FOREIGN_KEY_CHECKS = 0;

ALTER TABLE t1 CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;

ALTER TABLE t2 CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;

SET FOREIGN_KEY_CHECKS = 1;

SET FOREIGN_KEY_CHECKS = 0

Query executed OK, 0 rows affected. (0.000 s) Edit

ALTER TABLE t1 CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci

Error in query (1833): Cannot change column 'hash': used in a foreign key constraint 'FK_t1_hash' of table 'test.t2'

ALTER TABLE t2 CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci

Error in query (1832): Cannot change column 't1_hash': used in a foreign key constraint 'FK_t1_hash'

SET FOREIGN_KEY_CHECKS = 1

Query executed OK, 0 rows affected. (0.000 s) Edit, Warnings