Mesh rendering problem on mobile (using Vuforia)

Hello!

I have an augmented reality project that uses Vuforia 5.5 and we recently upgraded from Unity 5.4 to 5.5 (can't use latest versions of both for now) and a weird problem popped up: meshes in AR scenes are rendered as if their material was set to "Transparent" instead of "Opaque" (which is not the case).

This results in very weird artifacts, with faces further from the camera being rendered on top of closer ones and whatnot BUT there is no actual transparency (I can't see things behind the mesh, but the own mesh "overlaps" itself, if that makes any sense).

I am not too knowledgeable in the inner workings of Vuforia or graphics programming in general, but my educated guess is that it has something to do with the z-buffer, but.... why? Is there any fix besides upgrading everything (which I know would fix it all, but again... not the best option right now)?