WITHOUT OVERLAPS does not work with prefix indexes

create or replace table t1 (a varchar(6), s timestamp, e timestamp, period for p(s,e), unique(a(3), p without overlaps));

insert into t1 values ('foo','2012-01-01','2015-12-31'),('foobar','2013-01-01','2014-01-01');

MariaDB [test]> insert into t1 values ('foo','2012-01-01','2015-12-31'),('foobar','2013-01-01','2014-01-01');

Query OK, 2 rows affected (0.021 sec)

Records: 2  Duplicates: 0  Warnings: 0

MariaDB [test]> select * from t1;

+--------+---------------------+---------------------+

| a      | s                   | e                   |

+--------+---------------------+---------------------+

| foo    | 2012-01-01 00:00:00 | 2015-12-31 00:00:00 |

| foobar | 2013-01-01 00:00:00 | 2014-01-01 00:00:00 |

+--------+---------------------+---------------------+

2 rows in set (0.001 sec)

MariaDB [test]> create or replace table t1 (a varchar(6), s timestamp, e timestamp, period for p(s,e), unique(a(3), s, e));

Query OK, 0 rows affected (0.235 sec)

MariaDB [test]> insert into t1 values ('foo','2012-01-01','2015-12-31'),('foobar','2012-01-01','2015-12-31');

ERROR 1062 (23000): Duplicate entry 'foo-2012-01-01 00:00:00-2015-12-31 00:00:00' for key 'a'