Unplugging the battery connector would not have caused any damage. In your description you say it is an LCD screen and these can be problematic and you should have purchased an OLED screen. If it is an OLED screen I would back the phone up and do an update or restore and see if the problem persists, if it does you will need to return the screen for a replacement. Before you do though I would make sure the top flex connector on the screen is seated properly and if the screen is losing image or backlight, you can do this by shinning a torch on the screen and see if you can make out an image.