I met the same issue, and final found the solution.
In the animation that I set curve, I have set the additive type → Local Space → Seleted animation frame, and I choose the first frame of the same animation.
After I changed the animation frame to another animation, the curve value become 1, instead of 0 before. I guess if I choose the same animation, the curve value may offset each other.
Im poor in English, I dont know if I make it clear. Hope this can help you!