bug or nah?

A knockup interrupted a dash, this is intended.

Where's the issue? You can't escape the ult.

Actually, seeing as there's not bug, this thread shouldn't even be here.

It's the exact same way that Rek'Sai is able to deny Lee Sin's execute on his Q. For Fizz it's very easy to see when the Ultimate is going to go off as you have a debuff and a visual countdown of "3...2...1" to help you out. Definitely not a bug, Knockups are intended to stop all mobility or they would have virtually no use against champions with any form of it as they would continue on their path.