I have the same issue on my Samsung SMARTTV series 8 and MAG256; by pressing the power button on the controler it will turn off the TV. And a few minutes later the MAG box will go from idle to back on again and turns the TV on again.
As well if you don't use the HDMI-CEC option you will not be able to control the TV with the samsung remote ... It's a catch 22 ... lose lose ... 👎️ Perhaps a software update may be needed for the developpers maybe?