I don't know if this is relevant to your question, but you need to use the "spline height adjust" tool so that the spline circles turn yellow. If the spline circles are white (before they have been adjusted) the object will go up and down with the terrain.
However, it seems that even after the spline circles are yellow, objects will still move slightly when the terrain is raised or lowered. I suspect this is a minor bug in Surveyor, but it can get quite annoying, as you are constantly having to re-adjust things.
Mick Berg.