UPDATE on RocksDB table with unique constraint does not work

CREATE TABLE t1 (a INT, b CHAR(8), UNIQUE INDEX(a)) ENGINE=RocksDB;

INSERT INTO t1 (a,b) VALUES (1,'foo'),(2,'bar');

UPDATE t1 SET a=a+100;

SELECT * FROM t1;

MariaDB [test]> SELECT * FROM t1;

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

| a    | b    |

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

|  101 | foo  |

|    2 | bar  |

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

2 rows in set (0.00 sec)

MariaDB [test]> SELECT * FROM t1;

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

| a    | b    |

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

|  101 | foo  |

|  102 | bar  |

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

2 rows in set (0.00 sec)