I saw that once in my base today too. I haven't quite figured out how that happens. I will do more investigation myself. :)
Was there any change regarding the winds or power generation? as though it might be something involved in some change in the direction of the wind, probably something that has not been applied completely yet, believing that if the energy generation is based on the speed of the wind, probably what is occurring would be winds with negative force making the blades of the windmill rotate in reverse and generate negative energy values.
I didn't directly change anything. Also yes you are correct that windmills are based on windspeed (or technically the windmills rotation speed which accelerates/decelerates when the wind speed changes). Wind direction only goes one way for now. I put a temporary fix that will stop the energy from ever going negative, but I need to see what is causing it