I successfully lost over 30lbs on OMAD and I was drinking coffee with milk daily, sometimes twice a day.
While the soy milk in your coffee is technically breaking your fast, if you are primarily doing this for weight loss, I wouldn’t stress over those 50 calories.
I gained all the weight back, so I am jumping back on the wagon, and I putting oat milk in my coffee every morning. It’s the one pleasure I allow myself.