If upstream went to the trouble of designing different icons it was
probably intentional.
I guess at least one argument is that the icon in the panel bar changes
to show the current volume, whereas the icon in the menu should be
static to represent "volume" in general rather than reflecting the
cu
Thanks for reporting this bug and help making ubuntu better!
This bug is an upstream one in Adwaita icon I guess, but still good to test:
- installing the gnome vanilla session (sudo apt install gnome-session), reboot
and switch to the "GNOME" session in GDM.
- you should have the Adwaita theme
