The short form of expatriate is expat, not ex-pat.
Nobody was formerly a Pat. Nobody is an ex-Pat. It follows that nobody is an ex-pat, either.
Ex here is not a prefix, like ex-copy-editor or ex-journalist or ex-hack, all of which you could remove to reveal the occupational categories that make this mistake promiscuously (copy-editor, journalist, hack). If we accept the Latin root with its variant ending, there are three morphemes in expatriate, none detachable.
The kind of people who commit this “error” fit the “typology” of writer who is so unsure of himself and “afraid” of his own copy that everything has to be set apart at a “distance” lest somebody complain. Surely X and P can’t rub right up against each other? Better put that “dash” in just to be “safe.”
The word is expat. It hasn’t ever been ex-pat and, despite language change, probably never will be.