race condition in letting clause

run this query 10 times: 

select a, b, c from default group by a, b, c letting a=1,b=a+1,c=b+1 order by a, b, c

Sometimes it returns:
[

{ "a": 1,"b": 2,"c": 3 }

]

And sometimes it returns:
[

{ "code": 4210, "msg": "Expression must be a group key or aggregate: (`b` + 1)", "query_from_user": "select a, b, c from default group by a, b, c letting a=1,b=a+1,c=b+1 order by a, b, c" }

]