Unexpected result with firstmatch=off,loosescan=off

create table t1 (a int primary key);

insert into t1 values (1),(2),(3);

create table t2 (b int primary key);

insert into t2 values (1),(2),(3),(4);

create table t3 (c int, d int);

insert into t3 values (1,1),(2,2),(3,3),(4,4),(5,5),

(6,6),(7,7),(8,8),(9,9),(10,10);

insert into t3 select c+10, d+10 from t3;

insert into t3 select c+20, d+20 from t3;

insert into t3 select c+40, d+40 from t3;

set optimizer_switch='firstmatch=off,loosescan=off';

set optimizer_where_cost=0;

select t1.a x, t2.b y 

from 

  t1 left join t2 on t1.a=t2.b

where 

  t1.a=1 and t2.b=1 and 

  t1.a in (select c from t3) and rand() < 0;

+---+---+

| x | y |

+---+---+

| 1 | 1 |

+---+---+

1 row in set (0.002 sec)