I'm on a Mac too, but was not using the trackpad. Maybe that was it ? I have noticed it from time to time, but not in a reproducible way.
I was thinking of showing the target milestone after completing a challenge, or adding a score, but that got cut for lack of time. It will let you know if you did under-par. But I wanted to keep a positive feel, so it doesn't shout at you for using more moves than really needed.