ALTER TABLE: DROP CONSTRAINT is ignored when ADD CONSTRAINT is also present

MariaDB [test]> create table a (id int primary key);

Query OK, 0 rows affected (0.016 sec)

MariaDB [test]> create table b (id int primary key references a(id));

Query OK, 0 rows affected (0.015 sec)

MariaDB [test]> show create table b\G

*************************** 1. row ***************************

       Table: b

Create Table: CREATE TABLE `b` (

  `id` int(11) NOT NULL,

  PRIMARY KEY (`id`),

  CONSTRAINT `b_ibfk_1` FOREIGN KEY (`id`) REFERENCES `a` (`id`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci

1 row in set (0.001 sec)

MariaDB [test]> alter table b drop constraint b_ibfk_1, add constraint other foreign key (id) references a(id) on update cascade;

Query OK, 0 rows affected (0.023 sec)              

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> show create table b\G

*************************** 1. row ***************************

       Table: b

Create Table: CREATE TABLE `b` (

  `id` int(11) NOT NULL,

  PRIMARY KEY (`id`),

  CONSTRAINT `b_ibfk_1` FOREIGN KEY (`id`) REFERENCES `a` (`id`),

  CONSTRAINT `other` FOREIGN KEY (`id`) REFERENCES `a` (`id`) ON UPDATE CASCADE

) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci

1 row in set (0.010 sec)