Before it was a kind of bug thing where if she jumped before the cc hit it would still activate and be stunned when the jump completed. But now she's able to jump while cc'ed?
Is this a bug or is this intended? I don't find it fair when I use my void spikes with cho gath and hit her and watch her jump away in mid air.
No, there is no bug regarding her jump. Here is what happens in a couple different situations and why.
Situation 1: You cast your Q under Tristana. Tristana begins her jump cast. Your Q erupts and launches Tristana. Because casts cannot be interrupted except by death or losing vision of the target, Trisana's cast finishes and she flies away.
Situation 2: You cast your Q under Tristana. Tristana begins her jump cast. Tristana finishes her jump cast and begins to fly away. Your Q erupts and hits Tristana. Because dashes, which is what Tristana's jump is, can be interrupted by movement displacing effects, Tristana gets launched and her jump ends.
You can see these happens with any dash that has a cast time.
It was never a bug. The ability always had a cast time and so if she casts it right before or even as she gets CC'd the jump still happens. Same way Ezreals Arcane Shift works.
Pretty sure that knockups are supposed to stop dashes and jumps
"A unit that is airborne is unable to control its movement, attack or cast abilities for the duration."
But because Tristana had "Mercurial Scimitar" 
she was able to cleanse your knock-up and she jumped away in one swift motion.
You cannot Rocket Jump OUT of cc... the CC is still applied when she lands. So there's no actual getting OUT of the CC.
