I have similar problem. I created C++ version of the Epic’s Blueprint Splines Track project. The generated track is visible in PIE, Simulate and Standalone Game modes (with collisions also working).
So I just tested a new blueprint based project in 4.15.3, and was able to the spline blueprints when packaged to Win (64-bit).
I noticed yours is a code-based project, and you created your own custom class if I am not mistaken? I packaged your project and indeed was unable to see the spline, but this might be expected since it is your own custom class.
What I did to get the actor to show up, is once you have your final spline shape, you can convert it to a static mesh by using the Merge Actors tool in the right-click menu with the splines selected. Would there be a way for you to reproduce this in a new blank project without any custom classes?