I have a similar problem. An iPhone 6. The screen went black. Replaced with another. It was ok. Then when refitting went dark but can still see screen. Tried adjusting brightness in settings, it brightened then went darker. Tried another sensor flex but no joy. Any ideas?
I would replace the screen and clean away deposits from the water damage. Once it is cleaned up and a new screen is fitted I expect it will be fine. I have revived many phones from near death from water damage. Just a little care is needed to bring them back. The common faults are either the screen or the battery. :) Hope this helps. Good luck.