I have a QN90A (that I wish I never purchased) and now the software keeps switching to the built-in TV speakers.
I have a 6 month old Onkyo receiver connected to HDMI 4 because it's the only 2.1 port so I can connect multiple sources for 4k 120Hz. I know it doesn't support eARC, but I've still been able to set the audio to HDMI receiver and have it work for months now.
After the latest update, I can change the audio to the HDMI receiver and get sound from my home theater for a few seconds before the TV switches back to the internal speakers.
Turning off Anynet/CEC is not an appropriate solution to this. Connecting the receiver to HDMI 3 for eARC but no 4k120 is not an appropriate solution to this.
Samsung needs to allow the user to indicate which audio outputs (and quite frankly, a lot of other settings too) that they want to use without the TV overriding those choices.
It would be nice if Samsung would fix these issues, but after the numerous other issues I've had with this TV and broken promises from their support, I don't believe they will.
If you are reading this and are still in your return window or haven't purchased anything from Samsung yet, run while you still can.
Samsung's audio routing and CEC implementation are a mess. It's not improved since I've owned this TV and seems to just get worse. The latest update caused a rash of people to report of the TV reverting to internal speakers on AVSForum.