Sorry for the delay. We were really scratching our heads at this one because we actually don’t use “North” when wind speed is zero. When the wind is zero speed, the direction is “NULL”. You can see this in the “rapid wind” data - there is no direction at all when the speed is zero.
However, it turns out we were including those NULLs as zeroes in the average direction calculation - that’s what introduces the North bias at low speeds (the more 0 speed measurements in a given observation period, the more North bias). We’ll fix that in the next firmware update!