I have a problem that all of my LinesVisual3D are not visible, when placed after an object with semi-transparent material (lowered alpha channel). All other 3D objects are visible when placed behind, but the lines are not. What is more, when the lines partially intersect with the object, only the part that is not intersecting is visible.
Is it possible to make these lines visible?
I believe this might be an effect of how WPF 3D handles opacity. I've had similar issues with other Visual3Ds.
Have a look at Transparency in WPF 3D.
Basically you have to add opaque and transparent objects in the correct order. Try adding the opaque objects last. I.e. add the LinesVisual3D you're having issues with, then add the things with lower opacity.
Disclamer: I have not tried your scenario with specifically LinesVisual3D, so it might be that there is a bug in Helix regarding that specific visual.