"spin-down-on-ac" flag in "xfce4-power-manager-settings" doesn't work properly: if you check this flag then close "xfce-power-manager-settings", at the next startup of the "xfce4-power-manager-settings" application, the flag is cleared. However this does not affect the behavior of "xfce4-power-manager" as the configuration file is still written correctly.
Thanks for your report, problem fixed, will go into the next release.