I was in a bot match, playing as Syndra, and another character was Jarvan. He casted his ult and trapped 2 champions in the crater. I hit them with Scatter the Weak (her E) with a sphere summoned up and it knocked them out of his crater. I only did it once, but it is something that should be fixed.

RECREATE: Have Jarvan trap 1 or more champions with his ult. As Syndra summon up 1+ spheres and hit the champions inside the crater. They will be knocked out of it.

Why shouldn't you be able to do this? You can escape Jarvan IV's ult via dashes and leaps and be taken out of it via knock-backs and grabs. Why shouldn't you be able to push them out?

Yeah, I don't think that is a bug just the way it is intended to work. Riven can even hop out using her Q skill. Not really a big deal I think.

Its a knockback, Vayne and Poppy dont knock enemies through terrain.
Also Syndra cant knock enemies through any other forms of terrain.

Well thats because the mechanics on Vayne and Poppy's skill allows them to stun champs based on whether or not they hit terrain. Its an either or for them. Also if you read the details for Vayne's condemn, it states, "Temporary terrain such as Anivia's Crystallize and Jarvan IV's Cataclysm does not count as impassible terrain."

while it isnt a bug i think it should count as terrain so the ult would be more viable at trapping more than a handful of champs without a dash or jump