Are you setting the start position to the full length of the animation before triggering the negative play rate animation? Otherwise the start position is 0 and it has nowhere to go.
If, say, your animation is 2.5 seconds, you’ll need to set the start position to 2.5 immediately before triggering the negative play rate animation.
In all honesty, I’d recommend just animating it in reverse for your case to avoid these issues.