I tried to say, you can’t do anything and I did not want to imply you could change this by editing the Python code of the Generator. A Python Generator can not create proper splines.
Thus @nimpsy’s workaround, where the Tracer creates the actual spline.
Not sure, I can explain it properly. It may sound strange, if you have no experience in plugin development for C4D. Please forgive, if I fail…
Basically every parametric object is a certain type of plugin. In the SDK/API these are called ObjectData or lets call them Object Generators. There are different flavors of Object Generators, for example for mesh objects (i.e. Cube or Sphere objects) or like it would be needed here splines. All plugins get registered within C4D during startup via a special register function. This register function has a parameter which defines the flavor, which is then fixed during the life time of the plugin (aka the run time of C4D). So, the flavor is a decision of the plugin developer and it can not be changed during run time. The Python Generator is just this, a Object Generator with “mesh flavor”, which makes use of the user’s Python code to generate an actual mesh. And that’s basically it. There is no Python Generator with spline flavor. And there’s no way this could be changed by the user.
So, if you use the Python Generator to create a spline, like you did, you are actually doing something not really defined. The result is a strange hybrid of a spline and a mesh object, which not many places in C4D are able to interpret correctly as a spline.
You’d need to ask Maxon for a Python Spline Generator. Or you could look for a plugin developer to develop it for you. If you should be interested, you are welcome to PM me.