I have faced a lot of icon of app not matched in docklike in panel, i have solved a lot of them by refer to icon path to /usr/share/icons/hicolor/scalable/apps.
but for that authentication window , i can't find it and i dont know if it is app or what it is , if it is app , i can do that with it.
I don't know with the authentication icon, but the one in the panel is for application-default-icon.svg
It is usually used for those who doesn't have an icon.
that is the default icon theme in zorin lite, so i'm sure all will face that icon of authentication window, also that window has icon as the top of window show, that problem happen in docklike plugin which come built in with zorin lite, if i remove it and used window button plugin instead of it or dockbarx(anther dock), i don't face any problem with icon in panel
I have found the authenticate window , it is with name polkit-gnome-authentication-agent-1.desktop
locatated at etc/xdg/autostart, and i have matched his icon.