Maybe it's on hotel mode.
adb shell media volume --show --adj raise
adb shell media volume --show --adj lower
Is your sound through a sound system or external speaker? If so, that's your TV sound bar. With the remote on TV settings lower the TV sound to zero, then at least it won't affect the sound.
In sound right there in that menu. Turn off CEC and itll stop screwing with your TV.
Does it move to 50%?, if so then it's a random issue affecting any Android TV box or television, though I haven't seen it change anymore in the last few days on my device, it usually did it when I opened YouTube and other app.
That means HDMI CEC is turned on ... Try turning it off