I made a post about this a while back in the unity site but without any answers, here I am.
Recently, my project was upgraded from Unity 5.4 to 5.5 and it seems this caused an issue with the meshes displayed in my AR scenes on mobile devices (it works fine in the editor though). Basically, the meshes do not occlude themselves (eg. the arms of a character that have them crossed on their chest are rendered even when looking from behind). I can achieve the same result in the editor by changing the material from "opaque" to "transparent", but I have no idea why it happens everytime after building the project.
IMPORTANT: I'm using Vuforia 5.5 instead of it's latest version (which is why I'm also not using unity 5.6). The only reason for this is the change in the pricing that came with version 6.