SUBSTRING_INDEX() returns incorrect value when index value is negative

Build tested: 1.2.3-1

MariaDB [mytest]> create table t3 (c char(10)) engine=columnstore;
Query OK, 0 rows affected (0.169 sec)

MariaDB [mytest]> insert into t3 values ('aaaaaaaaaa');
Query OK, 1 row affected (0.165 sec)

MariaDB [mytest]> select c, SUBSTRING_INDEX(c,'a', -3) from t3;
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1 row in set (0.061 sec)

The query should return 'aa' instead.