It may just be my imagination, but I feel like my jump can't clear several of the walls that it should be able to clear. I understand that kha'zix's preupgraded jump is significantly lower in range, but even with it upgraded i still can't pass several walls.

Hard to give a 'duplicate' narrative, but I get stuck most often along the corners of a wall, and dragon's wall seems to block me too from time to time.

Has anyone noticed this, or is it my imagination?

I had thought that this was part of a mechanism... If you jump too close to the wall you can't get over it. However if you jump further away from the wall, everything is fine. But then again, I have no idea. Maybe it is a bug.

Well it is a bug because this happens to me almost every time i play Kha'Zix and i allways die if im being chased by people.

For Example,

I was running from 5 enemy players and i tried to jump the wraith wall but then he just jump in one spot and fell back down then a few seconds later i died.

It is because of the function of Kha'Zix's leap. Much like Tristana's Rocket Jump, his Leap is a dash ability as opposed to a blink. This means that the targeting circle for where you land has to be completely on one side or the other of an obstacle. If that is not the case, your character remains on the same side of the obstacle it was originally.