There are actually at least a couple of different bits of knowledge that Prospero withholds from various members of the royal party at various times in Act V. So I am not sure which one you are asking about.
At the start of the act, for example, he has concealed from all of them that he is actually Prospero, whom they know. Later on, Prospero conceals from all of them (and especially the king) the fact that Ferdinand is still alive and well.
But I think maybe what you are thinking of is the fact that Prospero does not tell the king about what Sebastian and Antonio have done. He does not tell the king that those two have been plotting against him.