From what I am learning...Yes. Any caloric beverage will break a fast, but the following supposedly don't break a fast.
Apple Cider Vinegar
Sweetners - Stevia, Erythritol, Monkfruit, Swerve, Aspartame, Sucralose (according to Keto Connect trial on Youtube)
Bulletproof coffee, Ketogenic coffee does break your fast...
Bee I don’t know for sure but a 50 calorie hit not breaking a fast sounds a bit optimistic from the info Ive found online from folks like Thomas DeLauer, Dr Berg, Dr Boz, etc, I think they would be a bit more cautious. A blood meter won’t tell you if you’re in a fasting state. That’s the one thing about Intermittent Fasting I don’t like as I do so enjoy a Bulletproof Coffee in the morning.
It also depends on what your goal is. I have cream in my coffee in the morning and I am fine with it. I then generally don't eat until noon or 1 pm.
I test my blood sugar and ketones in the morning and on a few occasions I have tested after having had my first coffee. Although my blood sugars were a bit higher, I still had enough ketones floating around to be considered in ketosis by that measure.
So does having cream in my coffee break a fast? Technically yes. Does it take me out of ketosis? Not necessarily.
So again the question becomes, what is you overall goal? To be technically in a fasted state and there by generating more ketones for energy, or just to avoid going out of ketosis or at best just for short periods of time while the excess glucose is metabolized and insulin levels drop again.