Robustness of facebook prophet

**Background**
I trained/tested a timeseries of 36 months in which I use 6 months for testing thus, training period is 32 months.

**Issue**
I performed two experiments on the same Data Frame with one single change i.e. I added - 0.0000000001 - randomly to the training months. To my surprise the forecasts came out very different. I did this with a small sample set and achieved an average of 23% differences in the sum of the forecasted values (take this number with a grain of salt). I've also seen cases with almost 50% difference.

**Question**
Why such a minuscule number when added to a training data value. Changes the forecast drastically?