x11-themes/adwaita-icon-theme-3.24.0 is installed and selected in xfce4-appearance-settings. However there are icons missing like Panel->Panel Preferences->Items ..all icons on the right side are missing like Edit or Remove. For instance Tango icons are working: x11-themes/tango-icon-theme-0.8.90-r1 Btw. there are other oddities, x11-themes/yasis-icon-theme-0.4.2 has the Panel dialog icons complete, but has missing all directory and file icons in Thunar. Reproducible: Always
You can investigate the installed files in order to track down the problem in the meantime: qlist x11-themes/adwaita-icon-theme qlist x11-themes/tango-icon-theme