Gameplay Rek'Sai

Stunning Rek'Sai when he is making his tunnels doesn't work, I don't know if it is a bug or it is how her ability is supposed to work. In the instance it happened to me, i was playing lux and tried to use my q, light binding on a half health rek'sai jungler who just used her tunnel making ability, my ability hit him but she kept moving finishing her tunnel, light binding wore off before she came back up right on top of me and she killed me. Otherwise i probably would have either escaped or killed him. Is it a bug or is she supposed to be immune to cc's when making tunnels?