I've noticed this behaviour as well, a private Discord server at least. Promptly changed passwords....Anyways, I actively click the field now to ensure focus, and then click the Login button manually instead of smashing Enter. Still, it's strange behaviour and I'm not sure if it's the XIV launcher or Discord, the latter of which seems to steal focus quite frequently on an otherwise inactive system. Maybe it has to do with notifications? IDK, I'm no expert but I don't like the behaviour.