I mentioned it because I have noticed that when using bluetooth as default audio that when I set the default volume at a lower level, after every reboot the sound returned to maximum, despite the entry 0.600000 in guisettings.xml remaining at the amended level.
I know that this has not been an issue for others and doing some more research I found references to a setting in /etc/pulse/default.pa that could perhaps cure the problem for pulseaudio where in addition to the lines:
Automatically restore the volume of streams and devices
There is a possible recommendation to add the line load-module module-volume-restore
But so far I am unable to make default.pa r/w in order to test it.
During my search I came across your thread and had wondered if the scenario was perhaps similar, so had to ask.
Fresh installs also seem to generally yield the same problem but sometimes in the past, when a new version has been released, it has worked for a short period.
So trying gradually to pinpoint where the issue may lie.