Ok, so Mark's coffee lore is getting intricate.
We know coffee effects witches differently. We know a stronger coffee can knock out a witch. We also know Amity enjoys coffee. We also know it isn't just a difference in how it is made since Luz made regular coffee for Amity and probs wouldn't have made it super different.
So what makes sense would be that Amity doesn't drink coffee to stay awake but to relax. Since the active compounds must be the same (since nothing else about the drink seems to change), caffeine must be a depressant for Witches instead of a stimulant.
Now Hunter has the effect of coffee as a stimulant. So either this is how Grimwalkers in general are affected, OR sine Hunter is a Grimwalker made from Humanity he is affected like a human.
Now we need Jay to drink coffee and have a strong reaction so we can know if it is a Grimwalker thing. Also, it would be interesting to see how it affects Demons, Titans, and Collectors.