Directional artifacts in MATLAB randn arrays?

I'm generating 3d fractal noise in MATLAB using a variety of methods. It's working relatively well, but I'm having an issue where I see vertical striping artifacts in my noise. I feel like it's a product of the original randn array that I'm starting with, as I see subtle vertical striping even when all I've generated is a field of randn noise, with no processing added to it.

I don't see the artifacts when I'm working on small arrays (eg <= 128x128x128), but my final product needs to be 1024x1024x1024, and the problem is quite pronounced at that resolution.

Does anyone have some insight in to this? Thanks!

Here's a link to the code used for the example image: https://github.com/MarcinKonowalczyk/randnd/blob/master/randnd.m