A bit silly, but you can’t add milk or sugar to already brewed tea or coffee. I feel like the crafting recipes for those things should include already made beverages rather than requiring you add it while brewing it.
Feel free to PR this.
As a suggestion: only make it available to those who do not have refined table manners trait, because no civilized person would add milk to the coffee made a while ago. Or make it available but incur morale penalty for the finished coffee. Disgusting.