Hi @Japhy I could add my input here and Giulio If you still need an answer on this.
It looks like you are trying to create spline thru points in the mass or adaptive component environment. Creating “elements” directly in a loadable family editor is different than most other creation methods. Below is C# code showing how to do it. I prefer C# since you are able to see an understand the “Types”. See the creation methods. This solution does not include element tracking and as things move so quickly make sure to check if spline thru points is now a component in RIR on the future builds. Either way this should give you some insight on how creation works in a loadable family vs the Revit env. Thanks