I can confirm this bug (notifications like pidgin or battery work perfectly but volume and brightness changes just show the empty black translucent box) and I can also confirm the workaround with inheriting the Human icon theme when using another icon theme.
I can confirm this bug (notifications like pidgin or battery work perfectly but volume and brightness changes just show the empty black translucent box) and I can also confirm the workaround with inheriting the Human icon theme when using another icon theme.