Try turning it off then back on. If that does work...
The problem has been solved on my setup so hopefully this will work for you. I have a 2016 UN49MU650DFXZA TV and new Q60T soundbar. Wasn't an issue with my previous M550 soundbar, just with the new Q60T one.
First thing to try is to use the "Universal Remote" option on the tv to setup the tv remote to control the soundbar. It doesn't seem like this should be necessary as all of the devices are Samsung but it did make the difference and the tv remote always works for the volume now.
I also upgraded the firmware on the tv by manually downloading the latest version from the website and using a USB stick. This latest version wasn't available through the tv's interface. I don't know whether this is necessary in addition to setting up the "Universal Remote" but it doesn't seem to hurt. And actually the TV Plus additional channels are working again. I did have to reset the Smarthub after this because the app icon bar got messed up but everything seems to be working fine now.
Have you also tried disconnecting the TV from the Soundbar and pairing it again by pressing and holding the source button n the right-side panel of the Soundbar for more than 5 seconds to display “BT PAIRING?
Be sure to click " ✓ Accept as Solution" when you find an answer that works for you.
Oh my god it's not just me! Did you ever find a solution for this? The CEC soundbar control mostly works - volume up always works, but volume down seems to get "stuck" at a random level....you can volume up from there but volume down always gets stuck at that same level until mute or restart. I'm seeing it on a Sony TV also (xbr-x930e) and a Samsung HW-M550.
And to the other replies...Yes I've tried multiple HDMI cables, yes anynet is enabled (otherwise CEC wouldn't work at all) and no, I have not tried re-pairing Bluetooth, since this is ARC over HDMI - it's not using bluetooth
