I've got the same issue on 1-week old 75MU6300FXZA. It's connected on the ARC port to a Sony HT-CT390 soundbar. The other connections are to an Xbox 1, and a Roku Premiere+. The issue does not seem to occur if I unplug the soundbar.
This thread was started 11-24-2017, still no responsible solution from Samsung. I also have the MU8000/55", no sound bar, no cable, no PS4. MU8000 hard-wired to Netgear Orbi system, sound out to Integra Receiver. Prior to Orbi network, MU8000 was doing the mysterious turn-itself-on. Appears that the 'official' Samsung responses on this thread are haphazard and basically clueless. I am joining the ranks of next TV will NOT be a Samsung.
Turning off Anynet+ on my MU6300 prevented the TV from turning on by itself, but if I turn on the TV it does not allow audio output over HDMI to the soundbar, it just shows options for optical and TV. I need to turn on the TV, enable Anynet+ and then reboot the soundbar for it to actually output ver HDMI.
Exactly same issue i have regarding hdmi receiver sound,sometimes loses connection and shows tv speaker or optical audio only, its a pain to get hdmi receiver sound back again, these televisions are a nightmare in digital form!!