Well after upgrading to 1.3.6 I get from time to time black screen now while using the phone.
I have adaptive brightness turned on.
Sounds like the same problem.
So I tried the following:
- adaptive brightness ON
- medium brightness
I went into a completely dark romm and the screen went dark.
Leaving the room again and the screen turned on again.
I disabled adaptive brightness and the problem doesn’t occur any longer.
So it seems:
ADAPTIVE BRIGHTNESS with MEDIUM BRIGHTNESS is a serious problem in dark rooms afetr 1.3.6 !!!
Workaround: Turn adaptive brightness off and never set brightness too small 
Question:
Can I savely switch back to 1.2.8 and if so, which methods should I use (update file or flashable image)?