Looks like /usr/share/icons/hicolor/icon-theme.cache is read only on panel start. So newly installed apps has some default/blank icon until panel will be killed and restarted. I didn't try strace or looking in sources, so its just a guess. Arch Linux xfdesktop 4.10.0-2
Closing bugs as xfce4-menu-plugin is deprecated/unmaintained/merged into Panel since years.