it seems that the GTK Stock Items have been deprecated  and therefore
newer icon themes (for example Adwaita) no longer include icons with the
old stock item names (like "gtk-go-down" aka GTK_STOCK_GO_DOWN used for
example in ). Since xfce-appfinder still use those icon names you
see, depending on the GTK settings and installed icon themes,
mismatching or missing icons in xfce-appfinder.
Fixed during the gtk3 port, more specifically in this commit:
Available on the latest development release (4.13.0).