DESC attribute does not work with auto-increment on secondary column of multi-part index

CREATE TABLE t (a INT AUTO_INCREMENT, b INT, UNIQUE(b,a DESC)) ENGINE=MyISAM;

INSERT IGNORE INTO t (b) VALUES (10),(10),(10);

SELECT * FROM t;

# Cleanup

DROP TABLE t;

INSERT IGNORE INTO t (b) VALUES (10),(10),(10);

Warnings:

Warning	1062	Duplicate entry '10-2' for key 'b'

SELECT * FROM t;

a	b

2	10

1	10

INSERT IGNORE INTO t (b) VALUES (10),(10),(10);

SELECT * FROM t;

a	b

1	10

2	10

3	10