Black screen on wake up but backlight is ok

Hello

I get an Iphone 6S plus with what I thought was a backlight trouble.

Iphone was correctly seen by itunes but no image on Screen. In fact at startup the screen was completely black even using a strong light. The phone correctly responds to power button, mute button but the touch screen was not active (I couldn't accept the connection with my Mac in Itunes and no way to slide to turn it off ).

I managed to restore it completely and it was still black. But when I reset it using power and home button, the phone waked up and was fully functional except a minor trouble , the SIRI mic was not working. I charged the phone all night long and it seems ok. When I decided to clean the microphone on top of screen to see if i can save it. After this, blank screen again.

Note: after some bad history with 6s I know that I have to remove battery before any screen connexion.

I proceed to a new restore and when again the phone was ok.

Now sometime when the phone goes in sleep mode (after 30s of no use) i can't wake it up. I need to proceed to a soft reset, and phone is back again.

During the 'black' session, the touch screen seems no active.

What could make such strange behavior ?

note: Screen is not a genuine, I didn't tried yet to change it

note: Motherboard seems ok, no water damaged indicator

Note: one time after a reset the battery was described as empty (red battery logo and request for connexion) but it was charged at 60 % (it was just after I replug it)

Note: sometime when i press the power button , the screen stays dark a few seconds then get brighter and brighter until it normal. as a capacitor was charging..

Any help is welcome.