Wrong result (NULLs instead of real values) with LEFT JOINs, mrr, join_cache_level=6

CREATE TABLE t1 (a VARCHAR(3), b VARCHAR(1)) ENGINE=MyISAM;

INSERT INTO t1 VALUES ('bar','b'),('aaa','a');

CREATE TABLE t2 (c VARCHAR(3)) ENGINE=MyISAM;

INSERT INTO t2 VALUES ('bar'),('ccc');

CREATE TABLE t3 (d VARCHAR(2), e INT) ENGINE=MyISAM;

INSERT INTO t3 VALUES ('fo',1),('ba',2);

CREATE TABLE t4 (f INT) ENGINE=MyISAM;

INSERT INTO t4 VALUES (1),(2);

CREATE TABLE t5 (

  f_key VARCHAR(3) CHARACTER SET utf8,

  f1 INT,

  f2 VARCHAR(3),

  f3 VARCHAR(3),

  f4 INT,

  f5 VARCHAR(1024),

  f6 VARCHAR(3),

  f7 VARCHAR(1024),

  f8 VARCHAR(1024),

  key (f_key)

) ENGINE=MyISAM;

INSERT INTO t5 VALUES 

  ('foo',1,'foo','foo',1,'foo','foo',REPEAT('foo',60),'foo'),  

  ('bar',2,'bar','bar',2,'bar','bar',REPEAT('bar',240),'bar'),  

  ('baz',3,'baz','baz',3,'baz','baz','baz',REPEAT('baz',250)),  

  ('qux',4,'qux','qux',4,REPEAT('qux',220),'qux','qux','qux');

set optimizer_switch = 'mrr=on';

set join_cache_level = 6;

SELECT t5.f_key, t5.f1

FROM ( ( ( t1 LEFT JOIN t2 ON t2.c = t1.a ) LEFT JOIN t3 ON t1.b = t3.d ) LEFT JOIN t4 ON t3.e = t4.f ) LEFT JOIN t5 ON t5.f_key = t2.c;

DROP TABLE t1, t2, t3, t4, t5;

f_key	f1

NULL	NULL

NULL	NULL

f_key	f1

bar	2

NULL	NULL

id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra

1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	100.00	

1	SIMPLE	t2	hash_ALL	NULL	#hash#$hj	7	test.t1.a	2	100.00	Using where; Using join buffer (flat, BNLH join)

1	SIMPLE	t3	hash_ALL	NULL	#hash#$hj	6	test.t1.b	2	100.00	Using where; Using join buffer (incremental, BNLH join)

1	SIMPLE	t4	hash_ALL	NULL	#hash#$hj	5	test.t3.e	2	100.00	Using where; Using join buffer (incremental, BNLH join)

1	SIMPLE	t5	ref	f_key	f_key	12	func	2	100.00	Using where; Using join buffer (incremental, BKA join); Rowid-ordered scan

Warnings:

Note	1003	select `test`.`t5`.`f_key` AS `f_key`,`test`.`t5`.`f1` AS `f1` from `test`.`t1` left join `test`.`t2` on(((`test`.`t2`.`c` = `test`.`t1`.`a`) and (`test`.`t1`.`a` is not null))) left join `test`.`t3` on(((`test`.`t1`.`b` = `test`.`t3`.`d`) and (`test`.`t1`.`b` is not null))) left join `test`.`t4` on(((`test`.`t4`.`f` = `test`.`t3`.`e`) and (`test`.`t3`.`e` is not null))) left join `test`.`t5` on((`test`.`t5`.`f_key` = convert(`test`.`t2`.`c` using utf8))) where 1