BKA join cache buffer is employed despite join_cache_level=3 (flat BNLH)

CREATE TABLE t10(a INT);

INSERT INTO t10 VALUES (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

CREATE TABLE t12(a INT, b INT);

INSERT INTO t12 SELECT a,a from t10;

CREATE TABLE t13(a INT, b INT, c INT, filler CHAR(100), key (a,b));

INSERT INTO t13 select a,a,a, 'filler-data' FROM t10;

set optimizer_switch= 'join_cache_hashed=off,join_cache_bka=on,mrr=on';

set join_cache_level = 3;

EXPLAIN  SELECT * FROM t12, t13

  WHERE t12.a=t13.a AND (t13.b+1 <= t12.b+1);

drop table t10, t12, t13;

EXPLAIN  SELECT * FROM t12, t13

WHERE t12.a=t13.a AND (t13.b+1 <= t12.b+1);

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

| id   | select_type | table | type | possible_keys | key  | key_len | ref        | rows | Extra                                                                              |

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

|    1 | SIMPLE      | t12   | ALL  | NULL          | NULL | NULL    | NULL       | 10   | Using where                                                                        |

|    1 | SIMPLE      | t13   | ref  | a             | a    | 5       | test.t12.a | 1    | Using index condition(BKA); Using join buffer (flat, BKA join); Rowid-ordered scan |

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