I found in my use of the immersion cooler so far that there is a bit of an overshoot in temperature when cooling. If I set it for 18C the fan will shut off at 18, but because the rod in the center has been cooled lower than that in order to chill the entire volume of wort, it will continue to cool the wort a bit more. Mostly a negligible amount. like 17.8C.
With this in mind, consider ambient room temperature and the effectiveness of the brewjacket insulation when aiming for a 3C mark. It's possible that the fight against ambient, even with brewjacket insulation, is leading the rod to be around 0C in order to keep the probe(assuming it's placed outside the fermenter) at 3C
I've noticed the top of the brewjacket is lacking insulation, which in most cases doesn't seem to be a big deal. But as you get further away from ambient room temperature insulation becomes more important. Maybe wrapping a towel or blanket on the top(while careful not to block airflow to the fan/heatsink and blowoff valve) would reduce the chance of this happening.
Just my thoughts.