I have a headcanon that it’s actually tradition for the Ravenclaw prefect to ‘not know’ the answer to the riddle when they bring up the first years for the first time. Because it makes the first years nervous so when the door asks a riddle (and the door makes sure not to ask such a hard one) the prefect turns around and says hmm, I’m not sure, and they all work on it together and it’s a lesson for the first years on teamwork and it also calms their nerves because they see that even the prefect doesn’t always know the answer