Hi! I looked into this and found out exactly why it's happening. To get the achievement right now, you need to click after the perfect zone, but before the bar completely disappears. If you let the indicator reach the very end and disappear, the achievement won't trigger even though the coffee is "weak".
I have already fixed this in the code on my end, but I can't push a patch to Steam right away. Please use this little workaround for now so you don't have to wait on me to get your achievement!