i use latest xfce4-session ported to gtk3 and i notice that when i change icon theme all icons in system will be change but when i click on xfce4-session-logout to power off or reboot my computer i notice that on this menu icons are not changed :/
Created attachment 7329 after change to other windows manager all icons works
so it was bug in xfwm4 maybe it help https://bugzilla.xfce.org/show_bug.cgi?id=13872 because it is second bug with xfwm4 where change to other wm solve my problem
Seems like you're issue is fixed already or the report itself was invalid for xfce4-session.