The sensors are inside the GPU and CPU die. There are no sensors on the motherboard that control the fans rpm. There is also a sensor in the chipset but that has no effect on the fans rpm.
What probably happened is that now with lower temps, your CPU is running faster. The GPU can now run closer to its full potential, causing it to overheat and throttle. You were CPU bound, now you're GPU bound.
Try undervolting both CPU and GPU. Also, install MSI afterburner or any other monitoring tool and see what the "limit" reason is. Don't waste your time guessing what's causing it, find the root of the problem.