When I shut down iMac (27" 2016 early version) a screen with distorted colours appears and shuts down. This does not happen every time I shut down the system. I think it appears when I do not shut down iMac for several days, instead I normally put it into sleep mode. I tried to take a photo of the screen but I could not as it flashed about 1 sec and turns off. I wonder whether this is normal when not shutting down iMac for a significant period of time or not. Anyone is having the same issue?