Play() does not cause SplineController to produce expected behaviour

[_Aka_]

Out of interest, is there a reason that on Initialisation, it would override any values supplied before it?

To be honest, I don't know. The asset was created by another developer before I acquired it late 2017, so I am not aware of the motivations behind many of the old design choices. Even when I find a behavior not justified or improvable, I often avoid changing it to not break users' code that relies on that behavior. I do sometimes choose to introduce breaking changes, but I need to feel that the benefits are worth it.
I do have a ticket regarding reworking the initialization process of some scripts (from memory: CurvySplineSegment, CurvySpline, CurvyController, CurvyGenerator, CGModule) to harmonize them and reduce/remove the frames needed for initialization, but it's not a high priority task.