Palpatine is a master manipulator who can also sense the currents in the Force. This does not mean he knows the exact future. He knows where he wants to end up, maybe even sees it. He's constantly changing and adapting his plans to achieve the end he forsees. This all started with creating a crisis on the world he represents in the Senate. When the Queen escapes, he changes his plan. He knew the Sith were going to have to come out eventually, so there was no real reason to not send Darth Maul to prevent the Queen from making it to the Senate.