Well I don't know exactly why, but a thing you might try if you haven't already done so..
Remember how models exported from max have there normals inverted, and to fix it you make a symmetrical copy. You could try this.
Also if you apply a texture projection in XZ, does the plain appear 90 degrees out? If so after you have rotated the model 90 degrees on the x axis after importing from max "freeze all transforms" , try your xz projection and it should be orientated correctly.
May help IDK. Best of luck.