system-log-out-symbolic and system-reboot-symbolic do not exist in Yaru. These icons are supposed to be used in gnome-shell when you search for "logout" and "reboot".
As a result, gnome-shell shows the icon application-exit-symbolic, which is basically an X, instead when you search for "logout".
Adwaita icons
Hmm. When searching for the german word "abmelden" (=logout), the "x" is shown. But it doesn't work with the word "logout" itself.
And I don't seem to be able to trigger "reboot" and "shutdown" at all (neither in English nor in German) . But that may be the symptom of this bug. Why is shutdown not mentioned here? Does that work for you? Or wasn't that working in the past and my memory fails me?
IMHO this together with the invisible extensions app are the most glaring Yaru bugs for 20.04 because they remove Shell functionality.
Shutdown is working very well for focal and now backported to bionic
Strange. What is the german word to trigger it? "Ausschalten"? "Herunterfahren"? Both?

In Bionic, I search for Ausschalten = Shutdown, Neustarten = Restart.
So many germans here! (german myself) xD
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So many germans here! (german myself) xD