I have recently built a Zynthian box using parts mostly sourced externally, as well as the kit v2.1 case and all-in-one module PCB.
When connecting the screen through a standard computer HD flat ribbon cable, practically the same as what is available for purchase from the Zynthian store, the screen goes completely white and stays that way. However, when the Pi is connected directly to the screen without using a cable at all using the built in female header, the display is fine.
I have been testing the connections for multiple hours between the Pi, the cable and both headers of the screen, and there is no visible issue with the setup.
I am using the recommended screen, a PiScreen 3.5".