ALTER TABLE gives false reason for refusing to instantly drop primary key

--source include/have_innodb.inc

create or replace table t1 (a int, b int, primary key(a)) engine=InnoDB;

--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON

alter table t1 drop primary key, algorithm=instant;

alter table t1 drop primary key, add primary key(b), algorithm=instant;

MariaDB [test]> create or replace table t1 (a int, b int, primary key(a)) engine=InnoDB;

Query OK, 0 rows affected (1.101 sec)

MariaDB [test]> alter table t1 drop primary key, algorithm=instant;

ERROR 1846 (0A000): ALGORITHM=INSTANT is not supported. Reason: Dropping a primary key is not allowed without also adding a new primary key. Try ALGORITHM=COPY

MariaDB [test]> alter table t1 drop primary key, add primary key(b), algorithm=instant;

ERROR 1845 (0A000): ALGORITHM=INSTANT is not supported for this operation. Try ALGORITHM=INPLACE