There's a difference between displacement and CC, which I believe is what's going on here. Basically movement cancels movement, but CC generally doesn't stop dashes.
Basically when executing a movement, dash or displacement the game issues move commands. New move commands overwrite old commands.
So in the case of voli vs J4, J4's dash move happens, then voli flips J4, issuing a new move command to J4 and preventing his current move from completing (and canceling the knockup portion)
In WWs case, he is immune to CC when he is dashing, but the dash ends as soon as he hits J4. J4 is already dashing though, so his dash continues under the suppression CC. Since his dash continues, the knockup effect still takes place, instantly canceling WW's suppression.
I mean, it's the same mechanic that allows trist or Ez to jump away after getting pulled by a blitz and hundreds of other strange abominations.
So it's not really a bug because the code is "working" as written - it's more of an unintended consequence of a 10 year old game engine.