Hi Luis!
The screen flash is common. It means that one of the screens aren't connected properly (most of the time) or the ribbon cable is damaged, so it think that it's not connected properly.
The screen that flashes is correctly connected, so in your case it means that the upper screen is at fault. Please check the connection and the ribbon cable if they are 100% good. If it's 100% correctly connected, then you might need to replace the upper screen.
Please let us know how it goes!