The minimum variance is 1 degree. My tank stays at 77-78. Do you have that setting to 1? If you do, then it maybe that your heater cannot keep up with the heating requirements with the lights off? Maybe need higher wattage.How do you have that set up? My Eheim and Ink Bird has a couple degree swings throughout the day and I can not seem to get it dialed in...
Without the controller in front of me, it's hard to visualize the settings but there is an option in it that lets you choose what the variance is IIRC.