http://www.youtube.com/watch?v=gZNdfVwkttM - you can see the whole problem described in this video if you do not see the image.
All the walls in all the images below have a translucent PNG texture. Each square wall, floor and ceiling tile is a separate GeometryModel3D (I know this is not good for performance, but ...). The floor and ceiling of the central cube do not have any geometry and textures, so they have the same color as Window.Background (black). But the effect in question is manifested in any way of obtaining transparency: texture for ImageBrush with transparency, Material.Color (for example, DiffuseMAterial.Color), where Color has an alpha channel, ImageBrush is a material where ImageBrush has opacity - I have the same problem.
All walls are made up of two triangles. Where there are no explicit normals, because I determine the indices of the triangle, so the normals are automatically calculated by WPF.
http://imagepost.ru/images/i/ma/image00001.png
It also has no back material or extra triangles on the back.
As you can see, there is no problem if you look only from + Z to -Z (standing on the blue square and looking at the red square is the second image).
But if you look back (from red to blue - the first image), there is no transparency!
Well, I decided to look from the yellow square (third image).
And then I came closer - you can see what is happening (pictures 4 to 6).
Geometry construction error or texture display error or lighting error! I guessed it was some kind of pruning! In addition, there are some interesting photographs 7 and 8 to prove your guess.
The last picture shows the white background of the window in which Viewport3D is placed (the previous one was black), and my assumption of clipping is confirmed - WPF just did not draw this part of the scene, and we can see the background background!
BUT! If this comes from different perspectives, why does the look from + Z to -Z (second image) look good ?!