It's happened the same thing, the screen don't turn off and all times have connector USB on but isn't true! I was gone to Samsung support and repair my Samsung a70, the diagonist was replace all circuit charge and replace cable type c.
Now my Samsung is back!
I have the same problem with my A70. Started mid December 2019. Screen won't time out and I can't make it go to sleep with the power button. Started out of the blue.
If it's plugged in, it works fine. Goes to sleep as it should both via time out and power button.
Recently as of 03Jan20 it will work normally periodically throughout the day. Then just when you think it's fixed, it's broken again.
Today -05Jan20 it has started to blue screen? Not sure. It does say its downloading a Samsung update.
Freedom Mobile- Canada
My phone has just started with this fault. The screen doesn't timeout automatically and pressing the lock button turns it off for a split second before it lights up again. In the notifications it is flashing usb connector connected and then disconnected. It just started doing this within the last few hours. Definitely no water exposure or dropping. Just a junk phone. Only bought it 2 months ago.