Erika
Check to make sure that the LCD panels cables are securly connected to the Logic Board of the iPhone. If they are verify that the cables for the LCD are not compressed or torn, they should gently fit between the frame and under part of the LCD display.
If the problem is still happening your digitizer is most likely the problem. Make sure this cable is secured to, sometimes especially on bigger devices such as the iPhone 8 Plus, the replacement digitizers glitch on outer edge movements. A great example unfortunately is the scrolling aspect.
Please type back with what you find good luck!