I'm literally playing SOTFS right now and ust tried to use Soul Appease on an Ironclad soldier in FoFG. It didn't work. I got right next to him and tried again, didn't do anything. It worked just fine on the regular hollows nearby, but not the ironclad soldier. It's the one on the wall by all the exploding barrels, right below the very first pursuer who drops down from a drake.