If you're looking for a math who will ignore the angle of the wall, just make a math like this:
Angle of entrance: 30 degrees;
Angle of exit: 180 degrees - angle of entrance;
But if you're looking into why the issue is happening, it's because the collision polygon is stretched when in isometric view.
While the correct is bounce and exit the wall considering the angle of the wall in the math, if you want ignore it and make the bounce happen like watching it in topview, just add the difference between the angles.
Using an isometric view angle of 45 degrees you'll obtain a bounce angle of 71 degrees between the entrance and the exit, while you want obtain 90 degrees, so, just add or subtract the difference to the exit angle and it should be sufficient for any entrance angle.