I see this a lot on iPhones where the component or components on the motherboard that control the video, or back light....Just burn out with the water damaged. Old screen may work on another phone, and a new screen still wont work on the liquid damaged phone.
So its very possible that a capacitor or something on the motherboard has burned out and would need replaced to repair it fully. As you can see from cleaning the board, the components are very tiny and would require special tools and serious experience.
Hope this helps