Malachi vs Malachy. One an Old Testament prophet, the other not. So certainty vs uncertainty in the prophecy arena.
You may find this surprising, but you'll get no argument from me there.
There are a lot of fake "prophets" floating around both in history and today, "Catholic" and non-Catholic. The point to remember is that there is no longer any public revelation binding on the Church Universal today. That ended when the Canon was established. That is why the Church even today does not compel Her members to believe in any present day prophecy (such as Fatima, for example). That is private revelation, even though some events associated with it were quite public.
And to be clear, St. Malachi is not considered a false prophet since the prophecies were attributed to him falsely. The forgers that made it up are the false prophets in this case.