ttk.Treeview.insert(... iid=None, ...) method has a check:
        if iid:
            res = self.tk.call(self._w, "insert", parent, index,
                "-id", iid, *opts)
        else:
            res = self.tk.call(self._w, "insert", parent, index, *opts)

Documentation says that "If iid is specified, it is used as the item identifier", but as you can see from code, iid is used only if it's "True". It means that you cannot use iids like 0, 0.0 etc.