Not sure you need the heat. I use a chest freezer and dual temp controller as well with a 2 degree variance. When the temp warms the unit kicks on and chills to set point as designed.
Perhaps adding the heat made the temps fluctuate more frequently and caused the unit to cycle more frequently than needed.
I have the chest freezer in the garage and here in Northern CA the temp can fluctuate to very warm/hot days to cool to very cold nights, so I have the fermwrap to account for the temp swings when we have an 80/90 degree day and a 50 degree night. Maybe I need to widen the variance.
I find heating during day and night swings isn't needed at all. In the freezer, with the amount of mass in the fermenter coupled with the yeast activity, the temp shouldn't really drop more than a degree. I only use heat when the temp is below my target for most of the day.