Sagging is normal for that type of cover when it gets snow and ice on it.
That sag does not look like a lot in the pic. How much did you drain down the water? Did the water level increase due to rain? You might want to lower the water level more in the pool when you get a thaw. So it will take more load on the cover before the sag hits the water.
Did you winterize the pool and install the cover or did a pool company?