FBX + vray proxy material problem

I figured out a problem when I import a fbx (or obj) file and the output is a vray proxy(any preview format).
It looks like the material (of the leafs) is only on one side of the face (on front side).
From the other side the face is complete transparent.
(Clear: in the rendering…)
I have trees and if the view is bottom up there are very less leafs visible, if the view is top down its like it should be and in the transmutr preview. From the side ist more less the half number…
I believe its the transparency…
That occurs only if tranmutr produce the proxy.
If the export (from transmutr) is a sketchupfile and I generate a vrayscene file out of sketchup after the error does not occur.
The problem is - the generated (out of transmutr) su files are very big in that way (about 800 mb) and it is not possible to import the files in su and export it in vrayscene.
So the proxies are essenrtial for me.
Any idea?

(sure, I import the generated su file)

Vray 5.0
su 2018
transmutr 1.2.6.