32 bit textures (alpha mapped) always disable the Z-Buffer (renders to front)
You have 2 options now:
1. Set your texture to 24bit
2. Apply this material with shader:
Code:
MATERIAL *mat_alphatest = 
{
  effect =
  "
    technique alpha_test
    {
      pass p0
      {
        zWriteEnable = true;
        alphaTestEnable = true;
        alphaBlendEnable = false;
      }
    }
  ";
}