I have two Samsung Q80T 65-inch TVs both connected to audio devices with HDMI ARC. One is connected to a Samsung HW-T650 soundbar and another is connected to a Pioneer AV receiver.
All ARC settings on the TV are enabled and everything is working.
The only issue is that the Samsung TVs do NOT turn on the soundbar or AV Receiver when powered on with Samsung remote control. All other features are working well and the TV turns off the soundbar and AV Receiver when powered off.
Nothing seemed to fix this issue, even after several calls to Samsung customer service and I had a feeling that only a new TV firmware upgrade could provide a resolution to this issue.
That is until I connected my laptop to HDMI 1 on one TV to test the TV as a computer monitor. Once I disconnected the laptop and powered everything off, the next time I turned on the TV with a TV remote control, the soundbar turned on also and started working as expected. I did the same laptop test on my second TV connected to a Pioneer AV Receiver and it also started turning on Pioneer AV Receiver with TV's remote control.
So, I can only assume that the laptop connection to TV's HDMI port somehow reset the HDMI ARC settings and all is good now.
Perhaps this will help others experiencing issues with HDMI ARC with Q80T models.