As a more avid Jarvan main than anyone in the entire world, I have more or less just accepted that his ult will always be a broken stupid piece of amateur programming trash that will never work as intended if it works AT ALL. Good Jarvan players know how to position that thing so that it doesn't matter if you can get out or not. That, by the way, is why you see us ult the support and not the adc or the tank and not the ap mid, it's because we want the wall where they are standing and when we are building tank, the damage is more or less irrelevant anyway.
However, if Riot wanted to actually fix it for once, they should make it impenetrable to dashes, but not blinks and teleports. People trying to dash through it should get bounced like they face slammed an Azir wall, but not interrupted in any way. So a Vayne attempting to dive roll over the wall would instead be directed around the circumference of the wall.
But honestly, it doesn't even annoy me how many people can dash out of my ult. Flash has always been a spell and Cataclysm has always been a spell used to make sure that you don't have flash when you want it. This is the way of things. But no, the bug that pisses me off the most which STILL exists is when champions who's only escape is to run fast manage to just power their way through the broken as fuck programming by sheer exertion of bullshit.