ExtractValue() with XPath variable references returns wrong result

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (a INT, b VARCHAR(10));

INSERT INTO t1 VALUES (1,'b1'),(2,'b2');

SELECT *,IF(@i:=a,ExtractValue('<a><b>b1</b><b>b2</b></a>','//b[$@i]'),0) AS xpath FROM t1;

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

| a    | b    | xpath |

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

|    1 | b1   | b1    |

|    2 | b2   | b2    |

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

2 rows in set (0.00 sec)

mysql> SELECT * FROM t1 WHERE b=IF(@i:=a,ExtractValue('<a><b>b1</b><b>b2</b></a>','//b[$@i]'),0);

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

| a    | b    |

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

|    1 | b1   |

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

1 row in set (0.00 sec)