Tangents ignored while setting keyframes

I am setting an animation curve in the Inspector to exist between the range 0..1. Then on each instance of the prefab using that curve to create a new curve scaled to fit within positionMin & positionMax (values of which are randomised on instantiation).

So I create a new curve, scale the values and copy over the tangents and time. Unfortunately the tangents don't copy over as they are set in the Inspector, but always flat. Even if I set them to wild numbers in this loop (I've tried 0, -10 & 10) all I ever get back is flat tangents. Is this a bug, or am I doing something wrong? Any ideas on ways to do this that works?

(in javascript...)

var keyframes = new Keyframe[anim.length];
for( i=0; i<keyframes.Length; i++ )
    {
    keyframes[i].value = Mathf.Lerp(positionMin, positionMax, anim[i].value);
    keyframes[i].time = anim[i].time;
    keyframes[i].inTangent = anim[i].inTangent;
    keyframes[i].outTangent = anim[i].outTangent;
    }
var newCurve = new AnimationCurve(keyframes);