I've had a couple of the 4K 700 series Sony models do this from last year. They freeze while trying to turn off (backlight is still on but no image) or will not turn off and won't respond to the remote or hard buttons. Myself or the client has to power cycle at the outlet and its good for a while. It's only been a couple of them so far and I like to blame it on the Android they baked in. The one Sony 700 series I've done in 2017 has been fine.

I'm tempted to do one more check for new firmware on the TV's and then remove the wifi access. I don't like recommending the smart features anyway...they just need a TV. I still like the Sony TV's though overall.