ALTER causes discrepancy in FK information between server and engine, unexpected ER_CANNOT_ADD_FOREIGN

--source include/have_innodb.inc

CREATE TABLE t1 (a INT, b INT, KEY(a), FOREIGN KEY(b) REFERENCES t1(a)) ENGINE=InnoDB;

SET FOREIGN_KEY_CHECKS= OFF;

ALTER TABLE t1 MODIFY a SMALLINT;

SHOW CREATE TABLE t1;

SELECT * FROM information_schema.innodb_sys_foreign;

SELECT UNIQUE_CONSTRAINT_NAME, TABLE_NAME, REFERENCED_TABLE_NAME FROM information_schema.referential_constraints;

TRUNCATE TABLE t1;

# Cleanup

DROP TABLE t1;

SHOW CREATE TABLE t1;

Table	Create Table

t1	CREATE TABLE `t1` (

  `a` smallint(6) DEFAULT NULL,

  `b` int(11) DEFAULT NULL,

  KEY `a` (`a`),

  KEY `b` (`b`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1

SELECT * FROM information_schema.innodb_sys_foreign;

ID	FOR_NAME	REF_NAME	N_COLS	TYPE

test/t1_ibfk_1	test/t1	test/t1	1	0

SELECT UNIQUE_CONSTRAINT_NAME, TABLE_NAME, REFERENCED_TABLE_NAME FROM information_schema.referential_constraints;

UNIQUE_CONSTRAINT_NAME	TABLE_NAME	REFERENCED_TABLE_NAME

TRUNCATE TABLE t1;

bug.trunc4 'innodb'                      [ fail ]

        Test ended at 2021-01-09 02:27:46

CURRENT_TEST: bug.trunc4

mysqltest: At line 9: query 'TRUNCATE TABLE t1' failed: 1215: Cannot add foreign key constraint for `t1`