Another thing that worked for me sometimes to solve this was running the overkill command in the drawing. You can even add this to your routine through some python, a good example here : How to use civil 3D commands in a Python Script? - #3 by mzjensen
But try what @Daan suggested, you can then get the length of your polycurve quite easily. I haven’t gotten problems with the node on the C3D toolkit