By the way you're describing it, I think it might be the way you E.
When you E a wall, you allocate your mouse like Flash. On a thin wall you can Flash over it easily but let's say on a thick wall, you can either flash over it if you're right at the wall but most importantly, your mouse cursor is at least more than 50% of the way across the wall so when you Flash in a sense you Flash into the wall but terrain makes you "clip" over to whatever side your mouse cursor is closer to.
Likewise with Talon's E, you can flip over thin walls easily, but when it comes to thicker walls, you still need to have your mouse cursor on the other side of the thick wall. For instance you can chase someone down by parkouring on the edge of a wall, essentially giving you a free gapcloser without actually going "over" any terrain.
If it isn't this then you're gonna have to describe it with more detail or even better, have video proof.