I did a lot of testing recently on this - not all tools appear to export splines correctly. Blender for example, I’m not sure how you export them so other tools can read them. Always came in as a list of points in every tool I tested with, not a spline.
Houdini doesn’t work though, I have passed this onto the dev team previously.
I’ve tried FBX and alembic from blender. Spent a full day on it but made no real progress. From your first and second replies I got the impression that the issue has something to do with how blender encodes splines into FBX and, that the problem is there.
The docs seem to indicate that splines can be imported from other dcc’s, however, if spline importing only works with c4d files that sort of limits the options a bit.