To reproduce:
1. Open attached project
2. Open "scene"
3. Open animation window and select the main camera. "Anim" should be the current animation
4. Open Custom > Open
5. In the custom editor window change the Length - instead of moving the keyframes, the script creates new ones. This does not occur for the 2nd clip (FOV), which does not have the position property

Extra bug: the animation window does not fully update after the length has been changed (only the length gets updated). You need to navigate to Curves and then back to the dopesheet - this shouldn't be necessary

Explanation: The problem here is that not EditorCurves were used. (AnimationUtility.GetAllCurves(...) is deprecated)