I think that
@Mana is talking about having a little soy MILK in her coffee, and not soy SAUCE, and my thought is that as long as you are not focused on a perfect fast, and you are still losing weight, then a little bit of some kind of milk or cream in your coffee is not a problem, unless you are drinking several cups and doing this all day long.
I don't like Black coffee either, and I usually add a little cream, but I don’t use sugar, and I only have one cup first thing in the morning. Sometimes, I even have bulletproof coffee, and that seems to help me not have hunger pangs even though I am exercising in the morning, before I come home and have my OMAD meal.