Until now I've been an ARC/HDMI/CEC unicorn. My Panasonic plasma worked faultlessly with my Onkyo, ARC worked, was always perfectly in sync, amp turned on and off with the TV.

With my LG OLED and my old Onkyo, everything worked just as well (I have the Apple TV and blu-ray player plugged into the TV and send the audio for everything to the amp via ARC).

With my LG OLED and my newer Onkyo (beefier amp and a REL sub), everything worked just* as well.

Until now. For some reason over the last few weeks ARC has been failing. We'll turn the TV on, starting playing stuff and no sound will come out of the amp. The amp still thinks it has an ARC connection, the TV still thinks it is sending audio over ARC, but nothing happens. Sometimes soft power cycling the amp fixes it. Sometimes hard power cycling the amp fixes it. Sometimes neither does, but the ARC connection starts working again 10 minutes later.

The TV is set to auto update the firmware, I suspect LG have broken something.

I'm bored with trying to fix it now. I've just ordered an optical cable, hopefully removing HDMI/ARC from the equation will fix it (though the amp isn't going to turn off with the TV anymore).

So, turns out everyone is right, ARC/HDMI/CEC is a steaming pile of unreliable non standardised crap (though it did all work flawlessly for me for about 19 years)

* ok, there was one new issue, the Apple TV when playing back 24p video would get out of sync with audio, but I managed to fix that