Immune to CC during the jump does not mean immune to CC during the duration of the ult. It's a long standing problem with auto attacks resolving if CC starts during the wind up animation, so Leona's stun went through because her auto attack was still starting up, and since WW was no longer leaping when the auto attack resolved, he was vulnerable to CC.
You could stop old WW ganks that way as well by launching CC just before WW ulted you in the direction he would teleport to after ulting. It's why you see in J4's champion spotlight video from several seasons ago cancelling a WW ult because he flag and dragged right as WW ulted him. It also meant Yasuo could stop a WW ult before the rework by having his third charge of Q up and using it just as WW ulted him, since it would resolve the animation and knock up WW after the suppression started.
What this means for rework WW though is that there is no third Q into the WW leap to stop the leap, since he'll just take damage but not be knocked up. It's the same as before where you have to time the third Q to resolve just as he ults you.