Query returns error, In Subquery LET,LETTING variables are treated as correlated

CREATE PRIMARY INDEX ON default;

 insert into default values ("k01",{"a":1});

SELECT f.f1

  FROM default f

 WHERE f.f2 IN (SELECT RAW s.f3 

                  FROM default s 

                   LET k1 = (SELECT RAW l.ff FROM default USE KEYS "key1" )

                 WHERE s.f4 IN k1);

SELECT f.f1 FROM default f WHERE f.f2 IN (SELECT RAW s.f3 FROM default s LET k1 = [5] WHERE s.f4 IN k1 );

"errors": [

        {

            "code": 5010,

            "msg": "Error evaluating filter. - cause: FROM in correlated subquery must have USE KEYS clause: FROM default."

        }

    ]

SELECT f.f1

  FROM default f

 WHERE f.f2 IN (SELECT RAW s.f3

                  FROM default s

                 WHERE s.f4 IN (SELECT RAW l.ff FROM default USE KEYS "key1") );