Maybe 1) the display is damaged 2) the display functions normally, but cannot receive information from the processor, some of the connecting parts must have problems.
As you have mentioned, it gradually become back, so it seems to be 2).
The phone functions correctly except for the display. I recommended sending it back for repairing. It will be very risky as you cannot determine which connector(s) are faulty, and it did not seem to be a software problem.