Can't move until ult is gone

This happened to me on only 2 champions so far but it doesn't just happens once or twice but lasts the entire game. When I'm use LB and ult, after I press r + the mimic ability I can't move. I stay where I am until the ult ability is gone. So if I press r and use LB's chains, I can't move until the chains are gone. Same thing happened with Viktor, I ult and I have to wait for my r to be gone to be able to move again