Why simple aql execute returns AsyncJobResultError when client requests data?

Question

Simple aql execute queries return this error too often.

arango.exceptions.AsyncJobResultError: [HTTP 204] job 4739915 not done

Sometimes it returns the data and sometime it doesn't while throwing above error.

Example query is below.

# here db is an instance of AsyncDatabase of python-arango library
# which is retreived by arangoClient.begin_async_execution 
cursor = db.aql.execute(f"""
  LET test = (
    FOR c IN TestCol
      FILTER c.status == @status
      COLLECT WITH COUNT INTO length
      RETURN length
     )
  return {{
    "test": test[0],
    "status": SomeStatus
  }}
""").results()

# to consume cursor
res = [x for x in cursor]