This is an artifact in `Graphics3D` rendering ([Z-fighting](https://en.wikipedia.org/wiki/Z-fighting)) which is generally difficult to avoid when using a depth buffer. 

While `ArrayMesh` is new in 10.4, you would see similar behavior if you did copy and paste the result into an older version.

As a possible workaround, try

    SetOptions[$FrontEnd, 
       RenderingOptions -> {"Graphics3DRenderingEngine" -> "BSPTree"}]