Scallable pixellation with a vector

Hi, 
I have a file where the vector is pixellated, the pixellation scales when resized. I can’t find how this is happening, it’s not a brush, FX, mask… 
It’s only this one file from this customer - I’d honestly love to know how this works/has happened to replicate it in future! (Or fix the issue in future) 

Thank you for your help in advance.