Bug : trees not displaying correctly (Unity 5.3.4p1 and 5.4)

Having recently upated my project for 1.3 (up from 0.8), I am having a strange issue with trees. Everything works ok in the Unity editor, but when running a build, the trees appear as all black flat single polys, or don't appear at all. The really strange thing is, when I create a build and test it multiple times, sometimes this same build will work be fine, and sometimes (most of the time actually) the trees will be broken (as described above).

I first observed this behaviour when I upgraded to Unity 5.3.4p1 (in order to support 1.3), I have also used the latest 5.4.0b15, with the same results. Previously I was using 5.2.3p3 (0.8) and everything was fine.

I am testing on a DK2, but I have a friend who has tested a build for me with his CV1, and he has observed the same problem (ie occasionally trees work, most of the time don't, all with the exact same build).

After searching around for a solution, I found some others facing the same issue, seemingly with no solutions :

  http://forum.unity3d.com/threads/tree-bug-in-oculus-build.390057/
  http://forum.unity3d.com/threads/trees-on-terrain-become-black-in-the-player-with-oculus.390146/

Is there anyone else seeing this problem? Any ideas/fixes/solutions?

Thanks in advance,

Ben.